[TMI]

You don't mention the Velodyne model. Those with external amps have a feedback cable to the accelerometer. There is a lot of gain here and it can be extra sensitive to coupled hum but this will hum with NOTHING connected to the input,

If that is not the problem, installing a transformer inside the amp with a TRS or XLR connector is a good way to break the ground loop from the preamp. Jensen is a very good source. They also make prefabricated boxes for the HT industry you can just plug in.

Try this test. Connect an AC milliamp meter (a multimeter in the MA setting with probes connected for 200ma) to the RCA jack outer ring on the Velodyne and the RCA plug outer ring coming from the preamp. With this RCA connected ONLY through the meter, turn the system on and measure the current between the two components.

Jensen has good instructions on how to terminate the shield through a .01 ceramic cap while using a twisted pair to the transformer primary on the amp side.