I believe the ULD series also came with a second RCA connect for the servo. If this isn't connected back to the amp, you won't get squat. You should have a SINGLE speaker lead from the ULD amp to the ULD sub. And a separate RCA connection from the amp to the sub, for the servo to operate and control the motion of the driver. I had a pair of the same a few years back, and if my memory serves me correctly, THAT's how it's done. I've been wrong before.
Velodyne ULD 15 connects to Yamaha DSP A1
The Velodyne has a separate amp, which has regular double "left-right" connects, in from amp, out to pre-amp. The DSP is a recent A/V receiver made for home stereo, and only has a mono line connect; I tried a splitter, to no avail. I'm stumped, unless someone has a suggestion, short of another thousand or so for a preamp separate.