Sometime in the next six months I want to upgrade to a subwoofer that will stay in my system for at least a couple years.  I've searched both the asylum and audiogon for people's favorite subs for music, but it seems like only a few options come up (Velodyne DD series, Vandy 2wq).
Due to all the upgrading I've done this year I need to keep the budget down to no more than $1k on the used market unless there's just a huge difference between what I can get for $1k used and something more expensive.  In which case, I suppose I could justify a little more on my credit card.  
 Thanks in advance for your recommendations!