B&W DM70's - top end lacking

I'd ditch them and get a pair of Infinity 2000A's. 12inch woofer, midrange cone, RTR electrostatic tweeters in a large box. Built from 1968 - 72 by Arnie Nudell! I have a pair I bought two years ago from the estate of a college professor in CA.     Near perfect condition, never abused!        Same design as your DM70's (but yours have been beaten up and will be expensive to restore!).

Thanks  roberjerman
I am not ready to ditch them yet.
The leak bass units work OK now and I am sure that the surrounds on the original bass units can be fixed.

I really wanted to know from an owner what to expect from the treble and if mine need to be redone.
The boxes are in exceptional condition and not beaten up at all, but I did not want to pay for an electrostatic rebuild if it was not required.

Hi Iscm

Well I never thought I would come across another owner of these speakers here. :^)

A few things I found for optimum setup and assuming your speakers are functional.

First are your’s version one or two? The recommended amp 25 - 100 watts. I would recommend at least 100 wpc due to that woofer. If using Solid State make sure its wattage doubles at 4 ohms as they dip to 4 ohms. The amplifier you are using appears to be 15 wpc - This is not enough to hear what they can do.
That appears to be problem one. You need more power.

Floor Setup
These speakers do not have matrix bracing so they MUST not be placed "as is" on a suspended wood floor. if they are on a suspended wood floor your description of

"the bass is overblown with complex orchestral or progressive"

is pretty accurate.

Solution.
They must be raised on a solid platform, to where the tweeter panel is level with your ears. This fixes two aspects.

It ensures the bass stays tight and clean, and the ESL panel is the most resolving. They like a larger space, do not put them up against walls. They should be free standing.  

Pic 21 on my virtual system shows how I had them setup.

B&W ESL DM70

I used them with and without external tweeters. If your room is heavily damped you may need external tweeters. They are the total opposite of B&W 801’s, in that they like an un damped room just like my Quad 57’s.

The thin curved ESL panel needs to be at ear level, but its shape does give a decent sized sweetspot.

Any ESL repair shop can fix the speakers if they need repairs.

Hope this helps. .

Hi  ct0517
Thanks for the reply.
These speakers are something special.
I have the original MK1's.

My room has stone floor (with thick fitted carpet) and stone walls.
I would say that the room is heavily damped and has very low ceilings.

The change of bass unit to the 13 in Leak Sandwich has helped quite a lot.
I was using a Quad 306 as my power, but that could not control the bass at all and in fact the Art Audio is much better in that regard even though it is not so powerful.
It has the option of Ultra Linear at 25 W with variable feedback
My volume requirement is not so loud these days.
One of the reasons for changing to the tube amp is just how much more musical the system sounds especially with Blues and Jazz.

I just want to tame the bass a bit and get the treble balance right.
Sometime I wonder if I have sometimes a voltage problem, as about 80 homes were just built near me in our small village.
The mains transformer did blow up about a month ago and when that was replaced I seemed to get louder treble.
Is it possible that I was not getting enough voltage to the Electrostatic panel? 
The treble sometimes seems to go down a bit at peak times.

ct0517
I see how you have raised your speakers and I did think about doing the same. On my speakers it could be done between the existing stands and the main box, say 6 inches (15cm) should do, I think.

It is strange, as this evening, most of the bass issue seems to be gone. The treble is right up which what makes me think about checking my mains.

Have you ever noticed your speakers sometimes sounding much better than at other times?

iscm -  I found the panels sounded best at ear level and by raising the speaker to achieve this, the bass improved as well. I have a device that gives voltage readout. Plugging components into it also shows how much voltage they are using. Any voltmeter can be used to check AC voltage on the mains. When it sounds good and when you notice a problem. If there is a difference that tells you something is up with your main. But if the mains voltage is good both ways, I would suspect an issue with the power supply in the cabinet so it should be looked at. That is not a difficult repair.      

ct0517. Tonight everything sounds good, although I still think that the treble is a bit recessed.
Did you make your own super tweeter?
I was thinking of using a ribbon crossed over at around 8 kHz with some sort of pot for adjustment. I presume that the effect should be subtle. 

Did you make your own super tweeter?

iscm

the one in the linked picture is a simple dome tweeter. On the (plus +) side of the line there are 6 or 7 resistors.  I add / subtract resistors to get the desired effect, if needed. Very subtle. The curtains that can be seen in that room can be slid open to reveal pine walls and drywall, removing damping as needed. I have tried both the Music Reference RM10 and Rm9 Tube amps with them, as well as an assortment of SS amps. They were quite a load for the Rm10. They sound great with the Rm9. Well, everything sounds great with that amp.

My DM70 are not hooked up right now but your thread is making me want to hook them up, upstairs. :^)

At 19:30 the sound seemed a bit off so I measured the mains voltage.
It came in at 233V.
I will measure again later.
These days 233V is in the centre or the range, so it will be interesting to see if it gets nearer to 240V later.
It may be that the power supply on my electrostatic units are a bit weak after all these years and exaggerate the effects of any voltage drop. I am not sure whether the 3000V polarising voltage applied to the electrostatic panels is regulated or not. 
Perhaps a change of capacitors is due here.

233V should really be OK.
Of course it may be a problem with dirty mains which can affect any good hi-fi.

20:45 - 236V
Tested again as sound seemed a bit better and more lively.
Could 3 Volts make much difference or could it just be that the supply is cleaner at this time?

23:00 - now 238V and sound seems a lot better.
That is near as dam it to 240V
Could it just be my imagination?

Now 23:30 and voltage bang on 240V
Best sound so far
It is like the noise floor has dropped and the treble is better defined.
They just sound less like speakers.

ct0517:
Have you ever had you treble panels repaired?
If so, where did you go and what was the difference?
I know about One Thing, but they are quite expensive and I have heard that their place is a tip. There is also a German man, but I have had mixed reports about him and he is VERY expensive plus there is international post.
My ideal would be somewhere that I could drive to easily from Somerset.
Friends who had Quad ELS units serviced have said that the sound comes 'alive' with the repaired speakers.

I may look at the energiser units as well, as I am sure that small fluctuations in mains voltage should not make so much difference to the sound. 
Perhaps I am being hypercritical as I know that higher end equipment can show up other issues and that all equipment tends to sound at its best late at night.

I have ordered some ribbon super tweeters and it will be interesting how they work.

Hi iscm

I acquired my DM70 ESL already refurbished.

here is a link that shows a pic of the woofer as well as an experiment with the Quad ESL. The experiment did not go well as the B&W crossover Woofer to ESL Panel is 400hz.

https://photos.app.goo.gl/jB2ocyZ1sMjUI86X2

It is using silver wiring.

If there are periods when they sound great, and other times when not so, that tells me the ESL panel may be ok and I suspect the ESL power supply / capacitors. So I would be tempted from your posts to have the capacitors / power supply checked out and recapped. I am pretty sure the DM70 are using electrolytic capacitors versus the polypropylene film capacitors of the 800 matrix series.

The manual recommends that they not be closer than 8 feet apart.

Owners manuals can be found here.

http://bwgroupsupport.com/manuals/bw-archive#ID_DMSeries

Service manuals can be found here.

http://bwgroupsupport.com/manuals/bw-service#ID_DMSeries

ct0517: The service manual is a great hep.
I had no idea that it could be accessed and as it is from 1970 there is a good chance that it is correct for my early versions of the DM70’s.

One thing that I notice straight away is that I have a missing pin A to one of my HF units. This is the Ground connection which is also connected to the centre tap of the audio transformer.
This may explain why I do not get an even treble across the whole panel. I could either get hold of a new plug or remove one of the redundant pins from the other speaker, as only 4 pins are used out of the seven.
First I will try to get a spare if at all possible.
The pin may have fallen out when I first transported the speakers. The seller was not keen for me to remove the HF panels, but I was not happy to transport them in one piece.

After the above I will check the electrolytic capacitors and the resistors.
I am working my way through the wiring replacing with Chord Odyssey as I go.
This is the cable that I use for the speakers and I use Chord for the rest of the system interconnects.

When you bought your speakers did the seller say anything about their restoration.

Thanks again

I just noticed a slight buzz in the right speaker.
When I checked, I could see that some of the filling was pushing very slightly against the cone.
I corrected this and now the sound it much cleaner.
These speakers rely on being totally filled, so it is hard to stop this happening.
In future I may try a different type of filling around the area of the bass unit to prevent this kind of ingress happening again.

I have also raised the speakers by two inches using two 2 * 3 between the speaker and the stand.
This has also made the bass much better (less volume and better definition).
Later I could turn the wood to make the extra lift 3 inches.

I did a basic test of the high frequency response and found that the cut off is around 10/ 11 kHz.
With a normal tweeter I can hear (just) to 16 kHz.

This may explain why symbols seem so recessed and why, depending on the music, things just don't sound quite right.

It could be the power supply to the panels or the panels themselves.
As my mains is just over 230V where I live, I could try setting the selector on the power supplies to 220V to see if the treble improves.

I suspect that it it is the panels themselves that need a refurb.
We will see

an experiment with the Quad ESL. The experiment did not go well as the B&W crossover Woofer to ESL Panel is 400hz.

ct0517: With the Quads, I would have thought that around 200 Hz would be better.
I also have a pair of Dali Skyline 2000 and they have a great bass with the right amp and they are totally open baffle right up to their 40cm ribbon tweeter.
They get around the bass loss by using a super efficient 15 inch bass unit which is attenuated until it crosses over to the mid range.
Dali claimed a flat bass down to 38 Hz.

When driven correctly, they have been the nearest thing to Quads will balls that I ever heard, which is what made me think about using their open baffle method with electrostatics.

ct0517: With the Quads, I would have thought that around 200 Hz would be better.

Hi iscm

A 200 hz setting is much too high for the Quads. the Quads are wonderful to 50 hz with the Music Reference RM10 which was designed for them.
they needed help below that for the bottom Octave only, and to help create fill /SPL in the room.

For the experiments I placed the DM70 as shown in the picture I linked,  and did not plug them in so only the woofer was active. The woofer plays to 400 hz as designed. This was way too high and the reason it did not work well. Did not mix well with the Quad ESL bass panels. 

In room 2 I have the 57's set up with two Dynaudio BM12s Subs. The subs come with their own Class A/B amp - 250 watts each - they are set to play 60 hz and down only. The RM10 is full out on the 57's.
The preamp used in that room has two direct outputs to allow for separate connections to the RM10 and the BM12s subs. 
So the subs are filling in 60 hz and down only.

I have the 57's set up with two Dynaudio BM12s Subs.

And how does it sound, as I have heard mixed reports about the results of subs with ELS57's?

Looking at the B&W crossover it should be very easy to split the high/mid and bass to allow bi-wiring or bi- amping.

I could then get the best out of my 15/25 watt tube amp and use my Unico  for the bass. The Unison Unico has 70 watt from MOSFETs and should drive the bass easily.
My Conrad Johnson has two outputs to make this easy.
Level matching could be done using the volume on the Unico.

I should receive my super tweeter today, but think that I should still check out the power supply.
I would have thought that the resistors should be checked first.
B&W claimed that they used NO electrolytic capacitors, but how long do diodes last.
Perhaps the B&W claim only related to the crossover and anyway the paper in oil's may also need replacing.

Anyway, I will do what I can bit by bit.

The Leak Sandwich drivers have already improved the bass after I removed half the speaker stuffing.
I checked the original bass units and see that they both need replacement surrounds but one makes a metallic pinging noise which is not so good.

I will send them both to Wembley Speakers to have them checked out.

Iscm
I have heard mixed reports about the results of subs with ELS57’s?

Yes. If your subs are across the room, near the main speakers, and one listens to different genres of music, high and low SPL levels, IMO, one needs to have remote DSP management. A remote control that allows you to control phase, crossover level and db levels from your listening position.
The Dynaudio brand version that I have looks like

this.

Once one has used a remote DSP you can’t go back to a Sub with controls only on the box.
I have detailed the Quad 57 and subs experience on my virtual system page and my Quad 57 review page. See my post here from the Quad 57 review.

https://forum.audiogon.com/discussions/review-quad-57/post?postid=1324755#1324755

The B&W DM70 ESL’s are probably the rarest known speakers I can think of in North America. I assume more common in Europe. Here is a speaker that cost more to make than what it sold for !
Unheard of today - B&W became a bean counter company (post - John Bowers era - RIP). They are definitely worth the time to restore.

The B&W DM70 ESL’s are probably the rarest known speakers I can think of in North America. I assume more common in Europe. Here is a speaker that cost more to make than what it sold for !

I was told that only about 2500 set were made in total including the Sony versions which were basically the Continentals (SS7000 - same as DM70c - curved front).

One of my friends has a pair and I have been offered another at a fair price complete with spare bass units (the original units need new surrounds). They have Leak Sandwich 13" bass units in at present (like mine)

There is also a pair of Sony’s (SS7000) on eBay just now.

I just took out the two power/ energiser units and was surprised to find that there were many voltage selections (250, 240, 230, 220, 115, 105) and that they were set to 250V.
When this was changed to 230V which is the nearest to my mains ay my current address, the treble units came to life and now produced output up to 14kHz. That is still not as it should be, so the next thing is to send the units to One Thing who repair these (Change all the diodes and whatever else may need doing).

If the sound is still not correct I will get the Panels repaired by One Thing, who say that they have made a jig for these.
I do wonder if the panels were last repaired with too thick a membrane and I do also know that if it is too thin there can be a bit of a gap between mid and bass. It may be for this reason that they always had a falling response above 16kHz.

Regarding the bass, I have now stuck a door mat to the rear panel of the main box (inside). This seems to have tamed a resonance in the upper bass, so now I will do the same with the other unit.

ct0517, I hope you are enjoying yours

Iscm

from the manual.

Impedance
Nominally 8 ohms but rising throughout the mid frequencies to some 20 ohms, falling at 20 khz to 4 ohms.

The manual specifies the following for frequency response.
better than plus or minus 4db - 50 hz to 15khz
plus or minus 5db - 40hz - 20khz

This tells me they need really good setup or people will hear 50hz - 15khz (or less) with an average setup..

The manual also states the ESL panel has a removable pad to control the rear radiation pattern. Make them more dipole. This would be very important for a full midrange and keeping them a few feet at least from the front wall. You don't have a lot to work with - its a small panel. Everything helps. 

My feeling is if B&W kept producing them, the ESL panel would have grown in size - taller, as ESL Panels become more efficient with size. Interesting that the Quad 57 treble & bass panels are vertical. The DM70 mid/treble panel is horizontal. From what I remember the DM70’s have a bigger sweet spot over the 57’s when the 57’s are kept on the ground on their stock feet.

Mentioned earlier I was thinking of taking them out of the stored spot and setting them up. But this needs my son’s lifting help so maybe when he visits. The power supply finding is interesting. Will check the setting on mine before they do get turned on again. I have multiple dedicated lines in the house and live in the country so the demand is not high - Its a solid 121 volts. So the 115 setting it is, and it is low.

ct0517:
This is what I have on the DM70’s so far from different sources.
https://1drv.ms/f/s!AsdR4rYZVpE4sUzCKIvZoEG9f5Ag
Sorry, but there may be duplicates and it includes what I got using the link that you provided

I like this in particular, as there is a very long explanation about their development:
https://1drv.ms/b/s!AsdR4rYZVpE4sDU7GJjlvhBf1_2N
I note that a later equivalent for 1974 is much shorter and more simple.

This tells me they need really good setup or people will hear 50hz - 15khz (or less) with an average setup

My treble issues also show up when I use an amp that previously worked OK with these speakers, so it seems that there is some issue with my speakers.
I talked to One Thing Audio who suggested that I get the ’energiser’ units serviced as a first step.
They said that it is very common for the diodes to fail and that they would replace them all as a matter of course. If that does not work I will send in the panels for repair.

My feeling is if B&W kept producing them, the ESL panel would have grown in size - taller, as ESL Panels become more efficient with size.

I am not so sure, as making the panels taller would change the dispersion pattern.
Their horizontal dispersion is also quite good due to the curve used.
B&W did say, however, that a vertical arrangement was tried.

Hope this is of interest
Ian

I have owned two pairs of these.One pair sounded a bit dull but the other pair had lovely extended treble so it really depends on their condition.

One pair sounded a bit dull but the other pair had lovely extended treble so it really depends on their condition.

I think that either my energiser units need an overhaul of the panels need redoing.
First I will get the energiser units serviced and if I still have issues I will move on to getting the panels done.

I did not think that the electrostatic panels should sound dull.

Hi, new to the site.
I have just picked up a pair of DM70. The ESL sound very good but I suspect that the energiser units needs some caps replacing.
The bass drivers have green felt DIY surrounds that are not helping in that department. If anyone can help in the sourcing of replacement surrounds I would be grateful.
I currently have them hooked up to a pair of Williamson KT88 mono blocks. They are rated at 20wpc. I have been using these on my Maggies and Apogees to good affect. Do I need more power? I have a pair of NAD 214 that are 500+ when bridged.

Hi, I should have clarified that mine are the Continentals.
I note the previous comment about suspended wooden floors, I will place them on some marble slabs.
I am located only a few miles from both the Stenying and Worthing factories though they offer little support to the very early models of which I have a number including DM3, DM4, D5 etc.
Best regards, M

I suspect that the energiser units needs some caps replacing.

dylanthecat:
After speaking to One Thing Audio, they suggested that it is normally the diodes in the energiser units that go.
They will, of course, do this work for you if you need.
The capacitors are NOT electrolytic in the DM70’s, so they are usually OK and One Thing said that the resistors are normally OK.
Apparently the diodes available at the time were not of a high enough rating, so tend to fail over time.

The bass drivers have green felt DIY surrounds that are not helping

It seems that the brother of the person who did mine had been active with your bass units!
I managed to get hold of a pair of 13 inch Leak Sandwich bass units in 8 ohm versions (most are 15 ohm).
These seem a good match in the DM70 cabinets, but will require four new mounting holes.
Wembley Speakers tell me that they should be able to redo my surrounds on my original speakers.
If you have the Continentals where the bass units have the curved front this may be your only option. (Some Continentals have a curved adaptor plate for the bass unit)

I currently have them hooked up to a pair of Williamson KT88 mono blocks. They are rated at 20wpc. I have been using these on my Maggies and Apogees to good affect. Do I need more power? I have a pair of NAD 214 that are 500+ when bridged.

Regarding power, that I think is personal.
I have an Art Audio Quintet which when in triode mode only gives me 15 watts, but seem to control the bass units well. Of course I can not get thrash volumes, but it still seems to go to realistic volumes with classical and most progressive.
When I tried my 70 watt Unico mosfet amp, I found that it offered very little extra headroom and no better bass damping, so I have stuck with my Art Audio.

Thank you ISCM,
I have the adapter plate with the EMI bass driver. I have ordered a pair of foams from the Netherlands that were the closest match I could find. Hopefully they will arrive today. Fingers crossed there.
I do have a pair of the Richard Allen high end audio bass drivers that I could try as a back-up.
I have two very good local chaps with huge B&W experience to look at the electronics. They will both have first hand experience here.
I need a new grill to the rear of one panel. Any suggestions for a source will be welcomed.
I brought these as spares not working only to find they just needed a few minor tweaks. I paid very little for them so are very happy with the purchase to date. Best regards, Martin

Hi Martin

I have the adapter plate with the EMI bass driver. I have ordered a pair of foams from the Netherlands that were the closest match I could find.

I do hope that the foams are the correct size, as most are for 12 inch and not 13 inch speakers.
The foams for 12 inch speakers were used for mine and caused problems.
When I get the chance I will get the correct surrounds fitted to my original bass units.

This is a repeat of the link from ct0517
http://bwgroupsupport.com/manuals/bw-service#ID_DMSeries
You should see that the energiser boxes contain the Energisers for the HT, the High Frequency Crossover and the associated transformer.
The Low Frequency part of the crossover is separated on the bottom of the cabinet.

I did wonder if it would be worthwhile making the speakers bi-wireable, as it would be quite easy to do.

Ian

I need a new grill to the rear of one panel. Any suggestions for a source will be welcomed.

One Thing should be able to help or advise here.
Mine are made of a slotted plastic.

Just to make it clear, I am not pushing One Thing, but I know of no one else in the UK with DM70 expertise.
There is a chap in Germany, but he is expensive.

I did see the speaker surrounds being offered as a kit for £35,
https://www.ebay.co.uk/itm/B-W-DM-70-hochwertiges-Lautsprecher-Sicken-Set-speaker-foam-surround-315/183148017101?hash=item2aa478f5cd:g:3xEAAOSwlStauOhM 

but in my case I think I need a specialist, as one of my original bass units makes a pinging sound and needs a total strip down.
Hears hoping Wembley Speakers can do this.

Hi Ian,

I have reformed the bass drivers with the Dutch oversized 12" foams. Though tight, they are just large enough and a great sonic improvement over the felt DIY surrounds.

I have removed the cotton gauze and all the damping. This has been replaced with a medium density acoustic foam on the rear panel and top panel. Lambswool quilting has been applied to the sides and based. A synthetic matting acoustic curtain has been hung directly behind the bass driver and folder over the base quilting.

The speakers have been lifted onto layered marble plinths with isolating silicon feet between each of the three layers. I have slotted four heavy duty rubber mounts (designed for hydraulic car jacks) under the speakers frame with isolates the metal base from the marble.

Very happy with the results, the bass has been transformed. The speakers sound better across the frequencies.

The speakers were definitely over damped!

Martin

Hi Ian,

I have ordered those other foams as a spare. Thanks for the link.

Have you changed your drivers over to eliminate the crossover?
Have you changed the speakers overs to eliminate the amp outputs?

Does it sound like metal hitting a dense hardwood block? I have heard of dry joints causing noises like this in bass drivers.

Best regards, Martin

Hi Martin

Have you changed your drivers over to eliminate the crossover?
Have you changed the speakers overs to eliminate the amp outputs?

So far I have made no changes to the crossover(s).
I only changed the drivers because my original bass units are faulty.
The first thing that I plan to do is get the diodes in the HT section changed and take it from there.

Does it sound like metal hitting a dense hardwood block? I have heard of dry joints causing noises like this in bass drivers.

One of the original bass units makes a metallic ping when it reaches full excursion.
This is why I am holding back on changing the surrounds myself.
I feel that it is beyond my expertise.
Just now, the Leak bass units work very well.
I friend of mine, who also has a pair of DM70’s, thought that mine sounded very good this last weekend and that is before replacing my diodes.

The speakers were definitely over damped!

I agree about the damping and now use only half of the original damping plus the door mats glued to the rear panel.
The original damping is now in three zip laundry bags per unit.
This keeps the damping from getting into the rear of the bass units and help stop the fibreglass from settling too much.

Lambswool quilting has been applied to the sides and base

Where did you get your Lambswool quilting, as I have had trouble getting hold of this kind of thing?
I would also like to replace the fibreglass with long fibre wool if possible.

This is my other thread:
http://theartofsound.net/forum/showthread.php?57264-B-amp-W-DM70-my-journey-so-far
and another relevant thread:
http://theartofsound.net/forum/showthread.php?50077-B-amp-W-DM70-rebuilding-an-Icon

Ian

Hi Ian,

I brought my DM70 through AoS as 'not working' for only £100.

I have had great difficulty getting hold of natural lambswool quilting. The material that I have used came from the thermal insulation that Completely Fresh previously used in their delivery boxes. It is perfect for my needs. Unfortunately they have changed the material now.

I removed 2 black bin bags full of glass fibre (I loath the stuff) from the cabs.

One of the problems when I first listened to them was that the damping was pushing up against the cone despite the cotton gauze.

I will upload some photos onto your AoS thread.

Martin

Hi Martin

How many grams of wool are needed per cabinet?
I have found a source at £10 per kilo.

Hi Ian,

Rather than 'stuffing' I have used quilting in 25mm thick blankets therefore I have been able to apply to the inside faces and as gravity drop acoustic curtains directly behind the driver. As a mass/ volume I would estimate that I have now only 20-25% of the original.

These speakers are now sounding very good indeed. Speakers, as we all know, are highly subjective and personal. The DM70 are currently in a room with my Magneplanars and Apogees and are more than holding their own.

Best regards, Martin

Hi Ian,

The other foams have arrived. They are the correct size. I will probably change them over at some point in the near future.

Best regards, Martin

I may have a go myself with new surrounds if I can sort out the ’pinging’ in one of the bass units.

Now I bought a sound level Meter and have been checking the frequency response.
There is quite a hump at 112Hz of about 8db.
Perhaps I could try adjusting the filling.
There is also a big drop at 60Hz.
Some more filling may help.

Hi Ian,

What meter are you using? I do have a vintage frequency generator but not a meter.

I know that there are several apps available.

Many thanks, Martin

Hi Ian,
I have downloaded a few apps that were recommended on the internet. They appear to largely correlate with each other.
I am not seeing any obvious concerns. There is a very minor dip at 35Hz though it is very minor indeed. Nothing to cause concern or requiring remedy I would suggest. M

Hello Gentlemen,
I too own a pair of DM-70s I am powering mine with mine with McIntosh MC30s.  The sound is amazing!  When I first heard them the bass seemed overbearing, but after some investigation I found the mounting nuts to be lose.  I tightened them and replaced the gasket on the back panel and all is well.  My bass units have been refoamed.
Cheers, Mike

For checking the frequency response I have been using:
http://onlinetonegenerator.com/frequency-sweep-generator.html
and
http://onlinetonegenerator.com/
to generate all the frequencies that I need.
I only have a basic sound level meter (£15 from Amazon), but it seems to do the job that I require.

Today my wool 'filling' arrived.
I ordered 5Kg, which seems to be more than enough.
Next week I will take out the last of the fibreglass and keep adding wool until the bass frequency response is much flatter.

This is where I get my wool filling:
https://www.worldofwool.co.uk/collections/british-wool-filling?sort_by=best-selling
I would say that 4Kg is enough for the two speakers.

It was put into these:
https://www.amazon.co.uk/gp/product/B01I95ER9M/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
and
https://www.amazon.co.uk/gp/product/B01HT2K1E6/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

I used a total of four laundry bags per speaker to contain the wool and stop it going into the bass units.
It also helps stop the wool from settling.

Hi. I'm not really qualified to be here because I don't know much about the technical side. Just want to say that my late father in law designed the case for the DM70 and I think it's beautiful. We have a pair of speakers, a central unit that I think is unique and the original Sugden amp that was used for the original demonstrations. The whole system will have to go to auction because we've downsized. Any ideas on the best place to auction it? I'd post a photo but I can't see that it's possible here.

May I ask how much you may want and where in the country you are located?

I would love to see a picture of the central unit.

You could upload photos to Google photos and insert a link (I think)

Update:

I finally put in refurbished energiser units into my DM70’s.
I fact I was lucky enough to buy a spare pare of DM70’s for £200 that needed new foams for the bass.
The energiser units from these ’new’ speakers have now had all their diodes replaced and I am now using them in my original DM70’s.
This has made a massive difference to my treble.
I then tried the treble units from my ’new’ speakers and found that the treble was even better and now matches the level of the bass unit.

My advice to anyone is, GET THOSE DIODES REPLACED.

I have ordered the foam surrounds:
https://www.ebay.co.uk/itm/Bowers-Wilkins-DM-70-hochwertiges-Sicken-Set-speaker-foam-surrounds-315/183148017101?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Soon I will put these surrounds on the bass units from my latest pair of DM70’s and install them in my original speakers.

UPDATE TO FOLLOW

Ian