want new sub/s and box for Sony HTIB

i have this 5.1 system. i got it for cheap, and they included a 5 yr warranty at no cost.

http://esupport.sony.com/US/perl/model-documents.pl?mdl=DAVHDX589W

the bass from the stock sub/box is pretty decent, according to specs its a 6 3/4 (i think it was 3/4") sub, 1.5 ohm and the amp feeds 280w rms to it.

id like to build a new box for either this sub, or get 1-2 new subs (6.5s or 8s) that i can wire to ~1.5, rather not go any lower becuase thats what the stock sub is at now.

if i do 2 subs thats 140w rms each.

ideas? maybe a t-line? idk anything about HT, but i want something that gets alot louder with the power im working with, and something that gets pretty low too. but its not just for movies, i use it probably 80% for music, so id like it to sound good for music as well.

also, suggest a sub/pair of subs. im lookin around on partsexpress now, its free shipping over 49 bux now, im guessing thats for the holidays