These folks make excellent speakers and offer an incremental increase in performance as your wallet gets lighter. psb
Your desire to get the most bang for the buck really depends on how important accurate bass is to you and how large an area you are trying to cover.

Try SVS.... http://www.svsubwoofers.com/subwoofers.htm

Outlaw Audio LFM-1. Havn't heard one but the reviews are excellent and can be read at the below URL. Sold direct at $579.00 with 30 day money back return policy if you don't like it. What better way to audition than in your own system. Definate Bang for the buck. Supposedly awesome with both music and home theatre. Co-designed by HSU research sub guru Dr. Poh Ser HSU.

http://www.outlawaudio.com/products/lfm1.html

For a little more money $749.00 the new REL Q108E is extremly compact but said to be VERY potent, down to 20Hz with both power and definition.

RG

Check out the B & W ASW 675 as well, that has a nice punch to it, comes with a 500 watt class D amplifier, 10 inch kevlar woofer, and it'll take you to about 22 - 25 Hz at -3dB.. impressive

It really depends on what you want. I like the outlaw lfm-1 for my movies and light listening. For serious music I have an external powered 18" sealed enclosure sub that x-overs at 80hz. It is a diy but simple to build total cost of speaker and cabinet $300 amp $150. Although it can take time to find an affordable 18" driver. It is worth the wait.