I bought a pair of Entec SW-5 subs (servo controlled) here on Audiogon for $700. Entec went out of business some time ago, but their subs were (and still are) very highly regarded. There were considered one of the few subs that were fast enough to integrate well with electrostatic and planar magnetic main speakers. I can tell you that they are very musical, and probably much better than anything you can certainly buy new for that price. They probably wouldn't make great HT subs, as there power handling isn't high enough for high excursion movements such as earthquakes and cannons, but for music they are fantastic. Although they don't come up all that often, you might be able to find a single for within your price range.
Good musical sub under $500
I need to replace my crappy old AudioSource sub, my listening room is 12'X14' x9', hardwood floors (livingroom, not sure if furniture such as couches effect subs in absorbing sound as it does with mids & tweeters) . It will be 70% music so a accurate tight bass is a must,. It will be matched up with my NAD T752 A/V and Paradigm Studio 20v3s up front. MY present Audiosource I've had for years has to be turned to full volume just to add some low-end (Crossed over at 80, anything higher and you get vocals and such coming out of it, really crappy as I said lol). I had ordered a Velodyne DLS-3750R earlier in the week(cancelled my order) the more I read up on them it seems they are rather boomy and velodyne's good subs are the higher end stuff which are out of my price range. Used is ok as long as it's from a reputable dealer or owner (ebay is definitely out, really don't trust high end stuff from there). Suggestions?