Ananth, you have a list of fine choices. Quite a dilemma to choose. The best price/performance is going to be the Hsu. I have this sub in my HT set up and am extremely happy with it. Works well for music, too. While the 350 is a capable sub, the Hsu will have more slam and deeper extension for HT and still be nimble for music.
The Axiom is probably the best stand alone performer of the bunch. You get good extension, clean sound, and no distortion. I guess the only bad thing I could say about it is the price. I have never heard the sub in person, but from what I have read here the 500 is easily competitive, if not better, than other subs out there in the price range. Another excellent choice.
Now, things get interesting--two subs! For HT, I am happy with one sub. In my two channel set up I have two and the sound is fantastic. The sound is much more balanced than with one, and the three dimensional soundstage makes the whole presentation seem "big." The downside to having two subs is that they are a booger to set up properly. Placement is very important, and as we all know, the place where the sub sounds best may not be the most decor-minded location. I believe mdrew has both Outlaw and Axiom subs. You might shoot him a pm and ask him what he thinks.
Don't worry about having too much sub for a room. Get as much sub as you can afford. Easier to dial a big one down than try to get a small sub to fill a cathedral.
Hmm, which one to choose? I don't know, each one sounds kinda fun.
