Which subwoofer?

+1 to @audiorusty on comments about JL Audio "e" series (I have an e110--superb sub for limited space) and separate crossover.

My previous sub, though wonderful sounding (SVS SB-1000) has a rather non-transparent, poor-sounding low-pass output crossover. Instead I got a very good quality Marchand electronic crossover and no longer relied on the sub's crossover. Big improvement, plus the fact is relatively few subs even have a low pass output (RCA pair), much less a good sounding one.

As it happens, my e110 is one such sub. But I found it more convenient to continue using the external crossover than the JLA's crossover output, due to a pesky ground loop.

If you don't happen to have an integrated or receiver w/quality filtered outputs for sub + mains, an external crossover is the way to go. 

@lewinskih01: Your question has me confused. After stating "So he's using F12's up to 40Hz", you then ask "Do you happen to know how high he crosses over to the F12G's?" Aren't those the same? Or am I not seeing what you're asking?

You're not differentiating between F12 and F12G, are you? The "G" designation is used in reference to GR Research, the only difference between the two models being the use of a paper cone woofer in the F12G, an aluminum in the F12. Danny prefers paper cone woofers, Rythmik's Brian Ding metal.

Danny uses the OB sub from whatever frequency is required by the loudspeaker it is used with (up to 300Hz), down to as low as it will play, which is into the teens (below 20Hz). In other words, with a low-pass filter on the sub but no high pass. When he adds a pair of F12's, he still uses the OB sub full range, without rolling off the bottom end to "make room" for the F12's.

Le me reiterate that the 40Hz figure I mentioned is the frequency I believe Danny uses the F12 up to when coupled with the OB. I may be mistaken, it could be somewhat higher, though certainly not above 80Hz.  

Perhaps a bit messier than integrated, I run two Scaena 18" subs with a pair of Crown 1502 amps and an old Maxdrive 3.4 pc. As was noted eariler, it is the DSP that is the magic sauce though.
My listening room is fairly large (17X32) and they fill the low end perfectly. 

@bdp24 

Thanks again. You are right I was misunderstanding what the models implied. I totally get the F12 since mine are F12-like in heavy DIY sealed boxes. I thought the F12G were paper cone and open baffle (like Danny's designs), but I see on Rythmik's site they are sealed. So let me rephrase and provide perspective for what I was saying:

I though he ran F12 up to 40Hz, xo to OB subs (his Kit 4 or 5) from 40 to say 180Hz, and then xo to another driver. I guess my thinking was biased by the way I set up my system. And I know another user who runs multichannel stereo and used F12s (to 50Hz) in such a fashion with F8s above (frequency-wise) to 175Hz and JBL M2 above, so I though Danny was doing this but on OBs. 

I totally understand why he does it the way you describe it too: below 40Hz he gets 4 sources playing (F12s and OBs) so a DBA. Worth noting Earl Geddes argues for 3 or 4 subs and says the 3rd and 4th subs don't need to be as powerful, which in Danny's case would be the OBs below 40Hz. Then Danny gets OB bass with servos from 40Hz to whatever he xo, and let a regular driver take it from there. Makes a lot of sense and would demand a redesign of my system because those F12Gs would replace my dipole 18" but would not reach as high and the smaller surface (2x12" vs 1x18") I guess would be perceivable. In my prior iteration I had 2x10" in the midbass and the move to 1x18" was a significant step up in impact of bass while also having dipole texture (of course an 18" in a box would have more impact than it in dipole). Maybe I need the 3x12" GR Research :-)

Good food for thought!

BTW: do you place your OB subs close to the side walls? In my case I would need to, maybe 4 or 8" from the side walls in the front corners, 2 or 3 feet away from front wall...not ideal distance to front wall, but space constraints. I wonder if being close to the side walls would further detract from them.

You really need several subs in any system no matter what the room size.   It is just a matter of which sub you decide on getting.    I have a lot of experience with many different subs in my listening area which is a very complex room to fill properly.  The area is not your more common rectangular room with a flat ceiling;  I have around 14K cubic feet of space with an 18 foot vaulted ceiling in the main section with overhead balconies, stairs and a cantilevered overhead room in the rear section adjacent to the main entrance which open.  There is a lot of glass on the eastern wall, mostly stone on the western wall with natural wood on both the north and south wall areas where the stairs are located going up to the cantilevered balconies and room.

After several years of testing multiple combinations of speakers and subs, I have this place really dialed in to the point it sounds like a concert hall.  I have about 7000 watts of power between my two mono block main amps and the various other subs strategically placed for wall location and spacing/phasing.   My brother who is a professional video producer and audio engineer said the system sounds the closest to anything he has ever heard to a real live concert (he has worked with a number of major national rock bands).   The key to this system is twofold:  great subs and great placement of the speakers with proper phasing.  Of course the main speakers are the most important part too.   

After extensive testing and trials, I settled in on a pair of Bryston Model T main speakers which have triple 8 inch woofers, dual 5.25 inch mids, and dual titanium tweeters and they are set up with the fully active input system, but I am not running the fully active electronics, instead I am using the super heavy duty passive PX-1 massive external crossovers and each speaker gets 2300 watts from a mono block amp.   

For subs, I have used a number of products (several of which have been spoken of in this thread) and I'd skip over all of them as I have found the Axiom Audio subs to be the best of the lot by a long margin.  First of all, they have the best drivers which are indestructible and their power supplies are all massive linear supplies with monster toroid wound transformers.  They do not use switching power supplies as every other sub I tried uses.  The dynamic head room is hands down way out over the others which makes for the cleanest, pure tight bass I have experienced.  These subs are what makes low end musical!  Every other sub I tested sounds like bottom of the lake muddy low visibility.   If you do not want to "hear" your low end, that is know where your subs are in the room, but wish to have just balanced, clear, full range music, you owe it to yourself to try these subs out.

My most beautiful sub by far of the lot is my Bryston Model T sub with its triple 8 inch drivers and an 800 watt linear amp.  This thing just sings with beautiful low mid range bass, it is built by Axiom Audio (Axiom and Bryston are the same company).  It has a gorgeous black ash real wood cabinet with incredible internal bracing, there are no resonances at all.... it was not inexpensive though, $5600 and unfortunately they no longer make this sub.

For you particular room, I'd look at the Axiom EP500 which is a fabulous 12 inch single driver sub and a 500 watt linear power supply (do not be misled by power ratings on subs as most rate their power with a switching supply which lacks the clean dynamic power you will be getting with low end music and a linear supply with a ton of capacitors and it will do a far nicer job).  I have one in the room and I just love it!   I also have the EP800 with dual 12 inch drivers.   All of the subs have the same amp design, just with slightly different power output based on the sub, but the exact same full range phasing controls which are a must for setting up and balancing the sound based on the room and speaker placement.

My advice due to extensive experimentation in a difficult area to properly fill with balanced sound is to go look over the subs offered by Axiom as this is their direct to consumer side vs. sold through full service dealers which is Bryston.  The Bryston speakers are the flagship items and will cost you twice as much (and the build quality reflects it), but the Axiom is priced very well for anyone looking to keep things within the reality zone and its also very top notch build quality.   Its all built in Canada, no made in China stuff here!   

That is my 5 cents worth :)