A single sub needs careful placement...and takes time to properly balance the gain and crossover settings...2 will both sound better and be easier to place...I started with a single REL S/5 SHO and it sounded great, but a second one added a year later was a nice improvement...

One possibility is that where you have the sub positioned is in a bass node in your room that’s sucking out the bass.  The first thing to do is trying different positioning to see if that helps, and it’s free.  Here’s the tried and true “crawl method” to identify the best spots in your room for sub placement, or generally a safe placement is placing the sub 20% the length of the room from the back wall and 25% the width of the room from the side wall if you can’t do the crawl method.

https://www.audioholics.com/home-theater-connection/crawling-for-bass-subwoofer-placement

If placement isn’t the problem, what you’re running into is that the REL is really a woofer and not a true subwoofer, which is likely why you’re missing bass depth/impact.  Getting another R218 may help a bit, but it still may not give you what you’re looking for and is expensive so if I’m you at this point I cut bait and get a Rhythmik F12G that’s about the same price, is very quick/musical, and will get down to 14Hz (-2dB) versus 25Hz (-6dB) for the REL. 

https://www.rythmikaudio.com/F12G.html

The other obvious option would be to get a bigger REL, but that will likely be quite a bit more expensive.  Hope this helps, and best of luck. 
 

Subwoofer/Low Frequency

Optimization see here

Mike

How do you have the REL connected? Maybe change the way you have it connected. Also, this appears to be a used sub, so make sure that it is actually working properly. 

Thanks for the responses. @soix I have not tried that crawl method but would be hard to do in my room. According to REL, the slope is rolloff a little to high. Rhythmik was one my option for a sub for some time.

@gkelly I have used high and low level connection and the gain is still the same level.

Hello @highend64 ,  something doesn’t sound right to me. I am a REL fan with a pair IN two systems. But even with one, it will easily play too loud. First thing, are you sure you have the phase correct?  I’m sure you know, but that is the first thing to adjust. You should easily be able to hear the bass go away and come in strong when the phase adjustment is correct.  If your REL  sub doesn’t have the ability to play too loud, then something must be wrong.

Hi @fastfreight I do have the phase corrected on the sub using a software from Rational Acoustics. The pros use this for phase and time alignment but for this is probably overkill. Honestly, if done properly you will hear it louder around the crossover point usually because of summation and that is what is great about this software. You can see the changes in real-time and not as a screenshot.

Thanks @highend64 .  Well I am stumped. While I wholeheartedly agree one sub is not as good as two, you still should get good volume and punch out of one.   For High level, you are tapping out of the amplifier outputs?  How are you connecting this as you have one sub and four amplifier outputs?  Have you tried just connecting to L and R outputs of one channel or one amp? (using  the Speakon connector and Red / Black wires?).

My REL r305 is very similar and I’m running it speaker level from a 40 watt Integrated amp and I only have the volume 5 clicks up from zero and a low crossover and it is plenty loud and keeps up with a pair of 88dB sens speakers easily.   Just a few clicks up and it is way too loud.  12x 14 room , well treated 

it almost sounds like it’s not getting enough signal from the amp   For troubleshooting purposes I would strip it down to its most basic elements, no EQ    Just the amp, speakers, sub , and a good line level source  

You could be sitting in a null. A larger sub will still have the null.  Also, move the sub around. Including into the corner.

Try the sub right behind your seat.

Small rooms can be problamatic.

@OP. I know you say the situation was different with a previous sub, but perhaps you are expecting too much of the Rel. There's only so much a sub can do when the main speakers are limited in frequency extension.

But even given that, there is no way that any Rel sub should sound insufficiently loud - even well below its maximum gain setting.

If you connect using the low level input you can use a set of test tones and an SPL meter to get an objective measure of the subs output. That will be of use to you in moving the sub around in case it is on a null point in the room.

+1 it's not the sub, it's the placement.  I've got at least one "suckout" location in my room that completely cancels the bass; sounds like you've already found yours.  Get one sub properly set up before you consider a second.

I have been going through the subwoofer research myself as I consider moving from a Paradigm sub that is a HT design, but not super musical.  The REL are ones I am considering. On REL’s page and elsewhere though I see cautions that when going to a more musical and precise sub than one for home theater use one may find themselves disappointed at the lack of boom.  The notes will be more accurate but the punch perhaps less. Maybe you are experiencing this. 

Need to tweak crossover 

I have a few RELs (2 in my main hifi rig) and even using various amps and speakers there's never been a gain issue. I use the recommended "high level" (from speaker outs) system and it works great. Since I've never tried the line level inputs on the RELs I can only imagine that's the problem, or not. It could be a curse or a bad REL or something to do with the relative humidity. Note that my RELs are all from the old "Q" series and continue to work perfectly, and were purchased used el cheapo.

something not right.

I sold a REL S/510 to a guy and when he connected it to his Amp (an AYRE i think) he could barely hear the Sub. After calling REL they finally advised him to switch the connection to the Amp (he was using high level) as soon as he did that "BOOM" huge bass.

regardless, you should be able to hear Too much bass with the sub turned up.

My experience in a small professional office (11X15X8 acoustic tile drop ceiling) I have two T/Zero MkIII, and at home I have settled on a T/9. I find that the bass is subtle, just like I want it. Yes, I have adjusted up to hear booming bass. But that's not what I believe a REL sub is intended for. Their HT (home theater) series I understand does that. But I find my RELs subtly complement my setup. But that's just me.  I don't want boomy. 

What @larryincmh  read in Rel literature may be your issue. I've had a number of subs over the years, low level inputs with dsp, I always heard these subs as separate from the main speakers, drew attention to themselves. My Rel's first sub I hear simply as extensions of main speakers, subs are completely invisible. Now I can turn up volume on the Rels and get much more sense of separate subs so volume, this with only a single hot wire connected, with both + and - connected even more excess. My take is either you have issue with connection, or more likely Rel subs may not be for you, Some people like to hear their subs, IMO this can be boom or excessively articulate bass depending on sub setup, can also be excessive impact and/or volume at certain frequencies, all these are artifacts of non coherent subs. On the other hand some like these artifacts in which case Rel not for them, hear plenty from those not liking Rel house sound.

Correct, most REL especially any with Speakon connections are meant to be an extension of the speaker.  They call it a Sub bass system.  With careful tuning it does no harm to the Mid bass and mids.  You're only supposed to hear that low when called for.   For home theater it's a different story.  

I have two RELs. Your problem is likely that this is not a punch and slam bass speaker. RELs are music audio system devices. That's why your getting the detail and textures of, say, bass strings or low organ notes or the skin of a bass drum.  This particular speaker was considered entry-level about 15 years ago. It's -6db at 25hz. You should be able to find a way to turn it up to a level that it creates boominess and sounds like it's stepping all over you speakers. To to that, turn the crossover all the way up to 120hz and you hi/low level control all the way up,  Should sound righteously horrible, but you'll hear it. Are you connected with the Neutrik Speakon connector? Check to make sure you haven't accidentally connected the black and yellow to the same speaker output on your amp.

Getting another REL will not give you the punch and slam. It's not how they are designed. I'd think Rythmik would be a cost effective solution for you. I've not heard one but do understand what they are designing their subs to do. I'd expect it to provide all of the punch and slam you could want, tho you will surrender some of that musicality you're appreciating. If this is a music system I'd suggest considering this as an evolution of your ear. If it's a home theater system then it's the wrong speaker. If your system does double duty then you've a priorities decision to make.

Good Luck!

I always liked the concept that you shouldn't notice the subwoofer when it's on, but will notice easily when turned off...

Is your 218 still under its trial period?

@surfcat thank you for your detailed explanation based on real life use of REL subs and it has helped me to decide whether it is even a good time to buy some new subs, or to continue to use my Paradigm’s for a bit while experimenting heavily with the settings on my kit.  My subs it seems are designed for HT but reviews say they actually are better for music than a lot of HT applications because they lack a lot of slam.  That’s OK by me as my system is used 90% of the time for music anyway, so perhaps I live with what I have and tweak.  I certainly don’t want things rumbling across the room, that is not how I listen.  Hopefully the OP gets their issue sorted out as well.

Have been through a progression on a number of different subs and several Rel models.  Totally agree with a number of posts that Rel subs are most compatible for 2 channel and not the best choice for HT.  IMO the best Rel for HT and music combined is the SE/SX series.  For my setup I have a stereo pair of Rel's for music and a JL Audio sub for HT LFE.  

Having said that, sufficient volume should not be a problem but will not sound great at max gain.  Could be an issue with a number of potential issues already mentioned?  Generally, the common mistake in setting up Rel subs is to have the X-over set too high and the gain too low.  Properly set up Rel's should add major "dynamics" to the music but be totally transparent.  Should not be boomy.  Comment above that the presence of the sub should not be obvious until you turn it off is right on.

Good luck in the quest...

Well thanks for that, Larry.  Glad I could help something today.  L0L

I’m not sure if anyone mentioned this before but there is a phase switch on the back. Flip it the other way. Hope this helps 

@sns His response is right on target.

The wiring is most likely the issue. I just put 2 T/9X's in yesterday and while they were in route I verified the proper wiring scheme to use. Here is what customer service offered:

"Thank you for reaching out. Either a signal ground or earth ground will be fine for connecting the black High Level wires to, though if possible the earth ground screw would be preferred. If you experience hum from the subwoofer when connecting the black wires to this screw, then you could instead try the signal ground and see if this yields improved results. 
 
We recommend starting with both the red and yellow High Level wires connected for each subwoofer, and in most cases this will result in excellent performance and usable signal levels. However, if you find that the subwoofers have too much output, where barely raising their Level controls already results in them overwhelming the output of your main loudspeakers, then you could try disconnecting one of either the red or yellow wire for each subwoofer. This will lower the input signal gain roughly by half, and will allow for greater flexibility when setting their Level controls. The red and yellow wires are functionally identical, though we generally recommend leaving the red wire connected for the right channel subwoofer and yellow wire connected for the left channel subwoofer in these cases."

The low level may be a better option for you. I found that I have very little volume needed for twin subs but am okay with that since it needs so little (about 30% of the volume knob) to mate with the Wilsons. Took me less than 10 minutes to get a good start to proper sound. I'll further tweak it this weekend.

Lots of good tutorials on their website and manuals are also available.

@jacobsrph And yes, check the phase. If sitting next to the mains then 0 is required Put your fingers on the main woofer and sub to verify they are working in the same direction while playing.

I appreciate all the help and recommendations. All of you have valid points and can't disagree. However, let me reiterate the situation I am having. With the Hsu sub I had had more of a "presences" feel to bass drum and bass guitar but still without the boomy and muddy sound I don't like. Perhaps maybe the Hsu sub was ported and has a lower response than the REL that I am use to. In the contrary, the Rel just disappears but in a good way. I feel it producing the full audio spectrum and not a sub/sat combo even though it is. 

I am actually using the high level connection and it wired properly. red to R, yellow to L and black to common ground on the amp chassis based on REL recommendation. As for the phase switch I have it 180. I have data to proof they are phase aligned. All you have to do is read phase traces on a graph interpret the slope angle. Again, I use Rational Acoustics for this since I have it available.

As of alot of you mention if one of the option was a defective sub. I really don't see or hear any defective issue with the sub. Maybe the gain staging in the Rel amp is not high enough but still is sufficient for my needs. 

Overall conclusion to this is that REL sub is sufficiently great as long as it produces clean, musical bass with detail and definition. 

@highend64 However, the gain is almost all the way up and I don feel that punch and slam I am looking for.

All REL sub-bass speakers are dramatically rolled off -6dB@rated output. Your HSU STF-2 is a -2dB@21 or 22Hz actual subwoofer which demonstrates what you're experiencing with their low frequency presentation differences. 

Not using the subwoofer crawl to properly position the HSU within your room could explain away the lack of detail you described.

Typically the rolled off nature of a sub-bass speaker simply cannot excite a rooms standing wave bass modes making room positioning far less, if at all critical. 

As an example: look at @wswright20's system photos showing his sub-bass speakers flanking the main speakers in the middle of a front wall, typically a rooms null and the worst place for actual -3dB subwoofers.

If you used the low level line connection from your preamplifier outputs with no improvement in gain I'd have the bass speaker checked out by a servicing REL dealer near you.

IMO, that extra low frequency Rythmik subwoofer option would be in play well before another over priced rolled off sub-bass speaker. 

I hope those nice folks on Addison in Berkeley are able to wiggle their way around those 52% tariffs on Chinese products. 

@highend64 I started with 1 REL T/9x and then went to a stereo pair. I don’t think you will necessarily get louder base, instead you get more of what you already hear.

Try unplugging your REL and mane sure your main speakers are in the best location to get to most articulate, fast and musical base possible. Then add in your 1 REL using the crawl method. Very small movements of the REL can make a big difference. For me, pointing the REL from near one corner, firing across the room to the other corner, was the sweet spot…..not placing the REL next to my main speakers “along the same plane.” 
 

Then add a second REL to enjoy to immense benefits of a stereo pair. 
 

By the way, I have a small room and use the high level connection with RELs upgraded Speakon cable.

hope this helps! 

BASS not base. 

@wolf_garcia BASS not base.

I make that mistake often and I'm a Double Bassist.

@labpro Then add in your 1 REL using the crawl method. Very small movements of the REL can make a big difference.

The subwoofer crawl is accomplished by positioning the subwoofer (not a sub-bass speakers) stationary at the listening position. The user then moves around the room noting and marking the strongest or best low frequency locations for subwoofer positioning.

Originally from Richard Edmund Lord and more recently from a REL brochure:

 [["REL subwoofers (I still like our original term Sub Bass System for its more complete description of what we do)"]] 

+1 on the Rythmik F12G.  I have one (one) myself and it's a great sub for music.  I have a dual system connected to it (home theater plus a 2.1 music system) and it excels in both applications.  I have no problem getting superbly musical, tight bass from it.  It's highly adjustable to that end (but does not have a DSP app associated with it - you'll have to do it manually) and digs deep.  Can't recommend this particular sub enough.

I would have been very interested in trying a Rythmik, but for those who don’t know this, the Rythmiks using the plate amps with high level connections (I think they’re the PEQ3s?) cannot be used on amps that have differential, or floating, outputs. Amps will go boom. In systems like this the Rythmiks must use the line level inputs.

Bass is both an instrument and a fish.

We are really talking about two different speakers here.  I had the same Hsu sub and yes, it was a beast, providing very deep bass with slam factor.  But, after messing around with it for a year I ended up realizing that I just did not like it.  The only way to get rid of the port chuffing was to bung up both ports.  Even then it was just too slow for my Snell D's and certainly for my LRS.  I never tried it in a home theater setup and I imagine it would be great for that. 

Enter the REL T9x and it was a whole different ballgame.  For me, the REL is much more nuanced and punchy, quick and 'musical'.  I will be putting another one in my system at some point in the future.  Or maybe try a different sub for flavor - Rhythmik perhaps?  But, not a Hsu.   

Anyway, I can totally see where you are coming form OP.  The REL will not do what the Hsu did.  Not in a million years.  But, I will say that I never turn the gain up on my T9x more than 5-6 clicks.  Even in my long 40 x 15 basement the T9x gives me more than enough.

Seems like the OP is telling us that he thinks it is operating correctly but is just a different kind of speaker than his prior sub.  I would say, however, that if the gain is at 100% there must be some issue.  That just isn't normal. 

I know this is basic, and I know we all know this, but MAKE SURE your  main speakers are in phase.  I bought a new integrated to take to my vaca home, and pulled out two cheap pairs of small speakers, to try it and compare.   I accidentally wired the second set backwards, with one speaker's wires reversed.  My o my did they sound bad.  I finally figured out what I did (embarrassingly), switched the wires, and the bass, image and everything improved.

@jji666 Yes, i am aware of them being different  sounding subs. They both are great subs but in different perspective means. As far as the far as the gain setting, it is not maxed out.  It happens to be at a little before 3/4. 

@fastfreight I have color-coded speaker cables. I would have to be color blind if that was the case.

I am in a similar position with the gain control near to max on my REL S812. The issue for me is that I am using high level and my speakers are very efficient (Avantgarde Duo 107dB/W). So there is very little voltage going to the REL.

In my situation it is ok because I have the REL situated just behind my listening position. If the sub were further away from the listening position then I would like more gain. I am aware of another Avantgarde/REL subs owner with a similar issue.

I had an SVS SB3000 sub prior to this and faced a similar issue.

I haven't tried the low level output from the pre amp. I'm not sure if this would make much difference as I only need a small amount on the volume control to drive the speakers loud.

I'm using modded Klipschorns, 103db have only one of the signal cables connected to each of my 300B monoblocks,  REL volume control only two or three clicks up from minimum. Two signal cables connected massively excessive volume. This with two REL TX9 in 13 W x 35L room, diy custom build 6' L sub cables, Speakon connectors single signal wire and ground to each sub and amp, ground wired to chassis screws not negative speaker binding post, bare wires at amp end.

I had this same issue with my Carbon Special. I could not get the high level connection working with the Ayre amplifier I was using. It turned out the problem was that I had the ground cable connected to one of the Ayre’s chassis screws (the foolproof way to connect high-level to a balanced amp according to REL). When I finally tried floating the ground cable (ie connected to nothing), the problem went away and now the sub kicks like a mule on steroids. 
 

Perhaps oddly, I didn’t have that problem with my Yamaha 2100 integrated when connecting the REL’s black lead to the Yamaha’s phono ground. It’s also a fully balanced amp, so not sure what the difference was. 

Another thing, you want that high-level cable cut as short as possible once you finalize your sub placement. It makes a difference. 

Lots of information on REL web site. I also think it’s maybe the wiring?

@helomech +1 This plus type of wire used makes a qualitative difference. I'm using 6' L of Duelund tin plated copper litz wire at present, have some VHAudio Airlock OCC solid core copper to try next, always use the authentic Neutrik connectors at sub end, bare wire at amp end thus far.

I have a single REL T2 (8" driver+ 10" radiator). 10'x15' room. I can barely turn on the gain knob or the bass is overwhelming. The notes about the phase switch and placement seem best. You can also play with the crossover point to put it higher so the REL plays more of the audible bass frequency-even though that is not the REL method. The notes about needing two seem less relevant to your issue. Good luck!

@wsrrsw @helomech what exactly do you mean by it us the wiring? Are you referring to the quality kf the cable or how it is wired. I have followed the recommendations that REL suggested.  

what exactly do you mean by it us the wiring? Are you referring to the quality kf the cable or how it is wired. I have followed the recommendations that REL suggested.  

How it’s wired. Try disconnecting the black ground cable from your amp. 

Try disconnecting the black ground cable from your amp.

Please go to their site (REL).