I'm fairly comfortable shopping for speakers and amps but subs, not so much. I've had a PSW10 for years, and recently got a Velodyne MicroVee as a freebie w/ a Denon 3313CI I picked up. The PSW10 rattles, and the Velodyne just doesn't hit (already sold it). So, here I am, picking a sub. Here's my current 5.1 setup:
C - Klipsch RC-52ii
L/R - Klipsch RF-62ii
SS - Klipsch Synergy B2 (connected via Rocketfish amps. They sound surprisingly acceptable)
Sub - temporarily using the Velodyne as, at least, it doesn't rattle like my Polk
Room is 18x16, seating is 8 feet from the TV, and the sub will end up placed against the front wall but not in a corner (can't be helped).
I don't want to spend over $500. From what I've seen, the SVS PB-1000 seems like a good choice and is $499 with free shipping on Amazon. I love Klipsch speakers and Fry's sells all things Klipsch about 25-30% below MSRP, so I could pick up a Klipsch SW-12 for sub $500 after tax.
I don't plan on spending another dime on this setup unless I decide to upgrade the surrounds and wire them correctly in the next decade - it's intended to last, and I want a sub I'll be happy with.