I own a ULD-15 with a walnut finish. Brought it used 12 years ago. The driver failed (buzzing sounds) I re foamed it but it still buzzed. Then the Velodyne company, and everyone I asked refused to accept the repair job. (It's the accellerometer they are afraid of)
I found an alternative (better sounding) driver, fits the ULD-15 series 1. I've enjoyed it for about one year so far. Its called an Adire tempest 15. I saw your post in the spring time but didn't want anyone to know how great my tempest sounds until I could get ahold of another one. (still looking for another ULD-15 "enclosure" for the my second tempest). Please sell me yours! I ordered 2 new tempest's today from creative sound who with some other dealers got permission to start selling (on a pre-order basis) the final batch of tempest's from the former USA factory. Adire is trying to re-start their business in Chile with new versions and higher prices on the tempest ($230). But selling the old stock subwoofer "tempest classic" at ($139 thats why I pre-ordered 2 more today).
As for the original velodyne ULD-15, I was in love with it, spent many late night listening sessions. Thought nothing could beat it. Then when I had to replace the driver with the ADIRE Tempest 15, I knew it would need 2 things to make it right (by the way this driver fits perfectly in the Velodyne enclosure). I wired the tempest's dual 8 ohm voice coil in parallel (now its 4 ohms). I bridged my hafler dh500 amp to drive the tempest (it only needs 250 watts but can handle much more) presently mine is 800 watts in bridged mode. The Tempest has long excursion and is design for a frequency response of 18 Hz to 250 Hz.
To get that response I used an old McIntosh MQ101 eualizer(inverts the natural bass role off to create a flat response down past 20 Hz). There are other simular products like the Marchand Basis "they are called linkwitz transforms". I also use a cross over (boxed my old DBx 120 and got a used Marchand XM-44)
I get rich, deep, powerful bass.
I sometimes listen to the ULD-15 with the Adire Tempest 15 for 5 hours on the weekend (no listening fatigue). The orig Velodyne driver was pretty good but wasn't easy to listen to for 5 hours.
The Adire Tempest "classic" can be pre-order until June 30, or July 4 at certain dealers. They will go fast (read the reviews, or any info on the Adire Tempest).
Good luck, and remember me if you decide to sell that ULD-15 enclosure.