question about what sub to buy?

The best real subs in that price range are from SVS and Hsu. These aren't mass-market brands, so your money goes to engineering and materials, not marketing and shipping from China.

Get one of these, these are true subwoofers, and will vastly outperform anything you can find at Best Buy:

http://www.svsound.com/products-sub-box-10nsd.cfm

http://www.svsound.com/products-sub-box-pb12nsd.cf...

http://www.svsound.com/products-sub-cyl-pc12_nsd.c...

SVS is also a great company, very customer-oriented. Email them and tell them about your room and price range, and they will suggest a model and where to place it.

Paradigm PDR-10 for $450 http://paradigm.com/en/paradigm/subwoofer-pdr-pdr1... . That sub kicks

Think about getting a Mirage OMNI Series 8" 400-Watt Powered Subwoofer. Best Buy sells it for $370. I looked at it recently. It features;

Stylish front-firing design with down-firing port

Delivers extended bass response to add depth to sound.

Built-in amplifier

Supplies 400 watts dynamic peak power, 100 watts RMS (continuous).

Front-mounted volume and crossover controls

For easy adjustment of settings.

Elliptical surround technology

Allows for larger cone surface, resulting in greater efficiency and creates identical cone movement to eliminate distortion. Proprietary surround ribs eliminate surround dimpling for controlled ouput.

Low-level and stereo high-level inputs

Provide flexible installation and connection options.

27Hz - 120Hz frequency response for faithful bass reproduction.

Best of luck with your decision!

a 100watt subwoofer will sound good. for best prices check ebay frist