Which Subwoofer would you choose?

Ditto for the used RELs...I have 2 I rave about here frequently and the first I bought was 200 bucks on Ebay (Q150e 10" front firing 150 watts and it's perfect) maybe 5 or 6 years ago, then a couple of years ago I found a Q108II (8" downfiring 100 watts) in mint condition also 200 bucks...both are superb, reliable things...and sound amazing. I make my own cables from Canare Star Quad wire and Neutrik Speakon plugs...spades are piled together on the speaker posts of my amp, and the main speakers use bananas so no problemo...very highly recommended.

In your price range, you may be able to find a REL Strata . . . They are excellent and great for a music system.  I have one and am extremely pleased with how it works in my 2 channel system!

I'm surprised no one has mentioned JL Audio subwoofers, which I believe are outstanding, quite probably the best -- at any price level, the Gotham (the most expensive), the Fathom (the second-most expensive) and the "e" series.  The subs are quite solid.  they are not boomy, and do not over-emphasize the bass frequencies.  JL Audio, in my humble opinion, and owning 2 e-112's, is the very best.

Look for an SVS 12 " sub used on craigslist so you can audition it before you buy.  Might be able to find an SVS SB-1000 for around $275.  Sells  on Amazon for $500.00.  Google SVS and read the reviews.  A Killer company.

I've had bad luck with used subs. Think I picked up 3 or 4 defectives off craigslist over time. The likelihood that a sub has been over driven and damaged is not insignificant. My policy now is new subs only.