Hum mystery - Crown K2 amp driving passive subs

Need advice. Just bought a Crown K2 500-watt amp to drive two passive subs. I will be running them through a dspmini crossover to augment my main speakers. Main system is Berning custom 6sn7 all-tube with tube rectified switching power supply, feeding Quicksilver v4 monos, into Verity Parsifals. Before I got the Crown amp I tried a 60-watt Rotel to drive the subs. No issues or hum with the Rotel.

Before I get the dspmini I was testing the amp. I am runnng it off the second pair of outputs from the preamp. With the Crown, the weirdest thing happens. When I turn on the Crown it hums through both speakers for about four seconds, then goes totally quiet. But the moment the slightest signal is put through -- whether music or just a little DC from rotating either of the Crown's gain knobs -- the hum returns. If I dont continue to apply signal the hum goes away in four seconds. If signal continues the hum continues until the signal stops. Again, this does not happen with the Rotel, wired exactly the same as the Crown. The hum also has some higher frequency noise, but mostly it sounds like 60 cycle hum, moderately loud. Too loud to live with.

Sounds like grounding? Would pro gear like the Crown be more susceptible? I dont imagine the huge power difference between the amps should matter. Any ideas? The Crown does not hum at all with inputs disconnected.

Thanks for any help.