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.
Its weird, the 4 seconds part. Try having everything plugged into the same outlet (strip, conditioner) but that's probably not it. More likely its a fault in the Crown. 

One thing I do know, you will want to run the mains full range and not cross them over but only adjust the subs for level and slope or EQ. Whatever tiny little bit you gain in detail or whatever from crossover will be more than offset by what you lose in bass response. Because with bass the more bass sources (locations) the better. Try it and see.
Yes the mains will be run full range.  I took the amp to a friend's house and plugged it into his system and there was no 60 cycle hum however his house has no third wire... two prong plugs only. But whatever noise such as to brush Etc came out of the speakers when no signal was present that noise completely disappears after 4 seconds.  now we are thinking maybe the crown is muting the signal.  someone suggested it has a muting circuit that is supposed to come into play after a few minutes of silence.  perhaps it's a little overactive. Having a tech look at it right now to be sure.  when I get it back I'll try it with a cheater plug again and see if that helps with the hum.  I have the DSP mini now and we'll hook up with the crossover.  crossover alone may eliminate all the hum.
Montaldo, I sympathize with you. Ground loops and other of their types of issues are such a PITA. I have a Crown XTi 2002 that I run in bridged mode to a 15" Tannoy passive sub, with no issues.

I agree with the cheater plug. You might want to look into transformer isolation units, that might help.

Best of luck,
The mystery is solved. Why the Rotel didn't hum may be because it has a two-pronged plug and perhaps didn't carry the ground hum through. In any case, the hum disappears after four seconds with the Crown because my friend and tech discovered that the older Crown K2 amps have a muting circuit that kicks in after just four seconds... to save power. So the hum was coming from my preamp, which has a slight hum. It was magnified by the K2's 500 watts. The the amp would mute itself.

So now I lifted the ground on the K2 and as expected, once I put the 48db/octave crossover in play the hum is no longer audible. There is also a small mod I will have done to the K2 amp so that the muting circuit is bypassed... because when it re-engages there is a slight transient sent back through my main speakers.

Thanks for the comments... just wanted to share what I learned. Will be keeping the Crown.