have you read reviews on the net, there are a ton of them and it could help you narrow down your choice?

2 makers who have an excellent reputation: Hsu and SVS;
there are others, but i can't remember right now.

if you have a good setup and calibration, you will never hear the sub, whatever you listen to, and the transition will not be heard;
it will be "invisible" to your ears.

hope this helps. :-)

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery