You also might consider the Audioengine subs. They have two models -- S6 and S8 (actually four models if you consider the wireless options for the above a separate model.) I have an S8 and was quite pleased with it for the price, though it has now been replaced by a GE Supersub X (at 3X the price.)
Inexpensive but decent 8" subwoofers
My current setup is Bluesound Powernode (N330) -> Blueroom Minipods
Which obviously isn't a high end system, but balances decent sound with sleek looks.
Looking for a 8" subwoofer which won't break the bank. Bonus points if it also looks good/minimal!