Your input please: Using sub on 2 channel amp

Go with REL. If you use speaker level it will add bass without changing the sound of your main speakers. Will improve the sound even if there is not a lot of bass present.

REL, Velodyne, JL Audio are all excellent choice for musical subwoofers. I have 15 x 20 room with Focal Electra fronts that can go down to 30Hz. The bass response is already pretty good. But when I added DD12 to the system, the bass response went up to the next level. You don't need to listen to bass heavy music. The subwoofer adds nice ambiance to any recording from small band jazz to symphonies.

ML Depth sub along with my 3.6 maggies sound great. Check out the ML line of subs while you are at it.

Lots of good suggestions above. Personally, and as a 2-channel, music-only guy, I can't imagine doing without my recently acquired REL Stentor II.

Forget the sub and look at amp/speaker combo. I've got a tube system with Dynaudio Special 25s (88db pigs). I bought a Dynaudio Sub250 to fill in the bottom when I watch movies; I can't remember the last time I turned it on. Try working on your speaker placement, it can make all the difference in the world. Also don't discount your speaker cables... (Oh and by the way I'm loving my VTL ST-150.)