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.
