I'd definitely look into fixing it yourself. Google is your friend. You'll learn a ton and gain some appreciation for it, save some money, and salvage a worthy older piece from the scrap heap. Win, win. Some new subs may be better, but they won't come cheap, and won't be that much better if you set it up well.
Velodyne DD15 - worth repairing the plate amp?
The amp in my 2008 Velodyne DD15 is dead as a door nail.
How good is this old marvel compared to today's offerings?
Worth investing another $500 (to have amp repaired, incl roundtrip shipping, tax, etc)? Or just use as passive sub?
How does it compete to modern subs, for example SBS SB-2000 or SB-3000 (both under $1k pre owned)?