recomended musical sub for 2 channel audio

I love my Rel Storm 3. Quadophile has written a GREAT review of the Rel Strata mk III:
Review of Rel Strata mkIII by Quadophile
-Bill

I second the Rel Storm III. I upgraded to the Stentor III recently which simply places the Organ "in your room", but short of flattening your chest with the last pedal note (as only the Stentor III can do) the Storm III is the best I have heard for pure music. I just opens up and spreads out the whole soundstage and adds magic to the mid range as well. I run full range but set the high filter ~ 35Hz or lower. A pair of Spendor S 3/5's or LS 3/5 a's and a Storm III could convince you that someone slipped a pair of pipedreams in your room when you weren't looking.

a pair of velodyne spl 1200r's blend very well with my system. are very tuneable and sound excellent.

Seconding ML, even Grotto is a real audio deal and reasonably priced compared to the rest of your set up. I demoed one with a pair of ProAc 1SC and had settled on the combo before upgraditis hit again. 22Hz is good enough for most of us!!!!

ACI Titan for sure. IMO, it is the most musical of all subs I've heard: Rel, Martin Logan, Klipsch, B&W, Sunfire Jr., Polk. It is also the best looking and most versatile: continuously variable phase and cross-over. And if you don't like it you can send it back for a refund within 30 days. You owe it to yourself to check it out:

www.audioc.com

And check out this review:

http://www.enjoythemusic.com/magazine/equipment/0405/acititan.htm

Do it. Get a Titan. I swear you'll love it.