The Axiom subs are great but I also highly recommend the SVS subwoofers. Amazing bang for the buck if you can spare the space (they are big and HEAVY).
http://www.svsound.comI've got a PB12-NSD ($569) paired with a Velodyne MiniVee and they create a 2-3 on the richter scale in my 3300 cu.ft. HT during movies. I find the sealed MiniVee is more musical than the ported SVS so during 2-channel music I run the MiniVee with the SVS turned off.
I realized I got where I needed to be movie-wise when my drink on the end table was pulsating like the cup of water on the dashboard in the Jurassic Park T-Rex scene. :-)
Another recommendation: get an isolation pad. I'm using the Auralex Great Gramma. Helps the sound a lot in a few ways. it reduces contact resonance between the sub and the floor. That in turn reduces rattling and also allows me to turn up the sub since it doesn't resonate through the floor to the bedrooms downstairs as much. I've got both subs on Auralex pads on top of carpeting with an 8 lbs carpet pad, and I can watch movies at reference levels at 2am without disturbing anyone.
