Best pre-owned subs below $1k?

It's about time I finally add a sub to my system, which I've been piece-mealing together. I'm a big believer in taking advantage of audiophile nervosa and grabbing a bargain on the pre-owned market.

As far as what I'm looking for, if there is such a thing, is the sub-woofer that provides the least (read idiot-proof) placement challenges. I should add this will be for an HT-only rig -- I have a dedicated 2-channel system in my living room.

The room itself is not a large one, say ~20' wide and ~11' deep. Because of varying reasons, my viewing position must be in the center of the backwall of this 20' wide room, placing me about ~10' from the system. My processor will be upgraded later (it's just an older Fosgate pro-logic processor right now), but my amplification consists of a 200w x 3 B&K amp for the front three channels, and a 200w x 2 B&K amp for the rears, all driving NHT VS-2s.

All that said, are there any no-brainer choices on the pre-owned market between $500 and $1,000? And, among those, are some less room/placement dependent than others?

Thanks in advance!