The way I see it the ULD was and still is a fine sub. The servo control system was a major inovation in 1985 when it was introduced. It continued to be one of if not the best sub avail for many years. But face facts. It is a 20 year old design, that once was high-tech.
It's a great unit. If you have one and it works, enjoy! If it doesn't work and you can get it fixed at a reasonable cost, do it. But, everything I read and hear leads me to believe that their new DD series is a significant improvement over the ULD. So if you have a ULD that doesn't work and you can't get it fixed cheaply, what could be better than a DD-15 for about $1200. By the way, when Velodyne was last repairing the ULD's they charged between $650 & 800 to replace the driver and tweek the amp. IMHO the DD series is worth the difference in cost.