I highly recommend the "slim" 8 inch subs by Monolith. They have 8,10 and 12 inch sizes you can slide under a sofa or wall mount. You can see the 12 inch (big) version mounted on my back wall if you look at my profile.
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!