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.
ULD 15 amp has a set of RCA "from" preamp and another set to "power amp." You may have gotten confused. If you did not change the crossover module, the ULD series came with default 85Hz crossover frequency. Make sure your Yamaha's bass managment frequency is lower than 85 Hz. Ideally, the Velodyne should be set at double the frequency of your receiver but default should work fine.

As long as you connect the single sub output from your receiver to either L or R input (from preamp) of Velodyne and turn up the volume knob on Velody accordingly, it should work. I have found having both L and R inputs supplied with two sub signals increases sub output.

Let me know if you have more questions.
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.