Which subwoofer?

Two Rel T5i's, a Rel T7x and a SVS SB2000. Fixed 90% of my low frequency issues.

@bdp24 

Danny Richie uses a pair of OB subs in the front of his room and a pair of F12G's in the rear (in both his company's listening room and at hi-fi shows), the F12's in phase opposite that of the the OB's. I believe he uses 40Hz as the x/o frequency between OB and F12.

This is great to know! So he's using F12s up to 40Hz. Do you happen to know how high he crosses over the F12Gs?I guess I have some chewing to do: I run F12s to 65Hz and from there unbaffled dipole 18", so kind of similar to Danny, but he has servo dipoles above the F12s. 

@oldhvymec
Not familiar with what VMPSs bass is. I believe you are also into DIY OB speakers. I'm also running an active system, so no passive xo parts between amp and corresponding driver. I couldn't be happier. Currently a 4-way active stereo system...although I smell bdp24 comments might be leading me to a 5-way ;-)

Has anyone tried the Thor's Hammer?

+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.