Lot's of talk on this! IMO the Velo DD series is hard to beat without spending thousands, so I say fix the amp. EBC Electronics in NJ does a fine job on them for a fair flat rate. Since this is a servo controlled design, no off-the-shelf plate amp will do...all advice to go that route should be disregarded. Turnaround time is the only drawback.
https://www.ebcelectronics.com/velodyne-dds-subwoofer-Repair-Service.html