SVS or Velodyne

Velodyne makes lots of different models/series. They perform very differently, one to the next.  It's been a while, but IME the large cabinet models are pretty good, the little guys less so.

Sealed SVS subs (along with Rythmik) tend to offer great measured performance for the $.  I replaced a pair of small Velos with a pair of 12" Rythmiks five or six years ago, and I'd do it again In a heartbeat.

REL would require some research, as I think they've substantially re-worked their older designs,. Those models were usually audiophile favorites, but performed poorly on deep bass tests. (For whatever that's worth to you - measurements mean different things to different folks).  The newer models may well test better, anyway.

htshack.com has a pretty comprehensive database of subwoofer test results if you're interested.

Neither- get a REL !

I have had Velodyne DD-15's for years. They have an on board EQ that shows you exactly what is going on in the room. The setup is not very difficult and makes the subs disappear. I have them set to 3 out of 100 on the volume. The biggest problem with subs is that people tend to crank them up too much.

Tim

I have a Rythmic 15" in a custom cabinet built by Salk Sound. Great, super tight base that really digs deep. Of course the cabinet is probably a 30" cube made from 1" MDF with enough bracing inside so you can set a turntable on top it and never skip. Weighs about 220 lbs too. Nobody ever accused Jim of cutting any corners building speaker cabinets. 

Those Salk built Rythmiks are great. You can also buy the DIY F15 kit and build an enclosure yourself, bracing the hell out of it as does Jim. The Salk wood finishes are really beautiful.