Subwoofer for Beginner

http://home-subwoofers-review.toptenreviews.com/bic-america-formula-series-review.html

A good sub is a universally usable component. Buying a cheap farter will only turn you off. A sub/s with good EQ will blend seamlessly and resale easier if need be.

http://app.audiogon.com/listings/subwoofers-velodyne-dd-12-digital-drive-subwoofer-practically-perfect-condition-2013-12-09-home-theater-90266-manhattan-beach-ca

I bought a REL Q150e on Ebay for 200 bucks about 3 years ago. It's an older model, but perfect except for missing the easily repaired/replaced grill (still had the frame). I consider it the among most fortuitous gear scores I've ever made since it KILLS, so to speak. Seriously...in my little system it is just an amazing sounding thing and all I do is turn it up or down a little here and there. All for the price of a very expensive sandwich or 100 rolls of nickels.

FWIW...I bought an older Paradigm PW 2200 sub for a song...these were close to 800-1k awhile back...incredibly clean and punchy...much better than their lower end PDR series...

Just a word of caution ... subs are speakers too! Don't cheap out too much. Matching a $200 fart thumper with your $5,000 main speakers might just work out fine, but if not, I'd blame the pedigree or implementation more than the concept. (For all of those who spent $200 or less on their subs which work wonderfully for them, please do not take offense. Not all $200 subs are made the same. I am not inferring about any specific sub mentioned in this thread.)
If not confident in your purchase, buy used so you can re-sell at little/no loss if you do not like.