I don't have that much space, and I'd like to tuck this away under by cabinet, so I don't have to run wires on my floor. I was looking at some compact ones like:
Soundmatters SUBstage100 (http://www.amazon.com/dp/B000JL4ODO/ref=asc_df_B000JL4ODO706964?smid=ATVPDKIKX0DER&tag=nextag-ce-tier3-delta-20&linkCode=asn)

or Yamaha's YST-FSW150YST-FSW150 (http://www.crutchfield.com/p_022YSTF150/Yamaha-YST-FSW150.html?tp=187) or FSW100, or their sideways one. The 150 goes down to 30Hz

Can't these really be good that small? I haven't seen many reviews on these.

I have a Yamaha YSP with a Celestion CSW 75 watt powered sub with 8in woofer goes down to 30Hz. It's about 10 years+ years old bought for around $400 back then. If this is going to sound better than the compact ones, I'd rather keep this old thing.

If you aren't adverse to a cube shape, I have been rather impressed with the Def Tech cubes. That little 7.5 inch cube puts out some serious power, not just for its size. They are expensive though. http://www.bestbuy.com/site/olspage.jsp?skuId=7037119&st=subwoofer&lp=3&type=product&cp=1&id=1151658131694

Totem (http://www.totemacoustic.com/products/sub-woofer/) makes some impressive teensy-tiny subs.