I just was looking for a similar noise with my active crossover. Lifted grounds, star-grounded, moved filter and bypass caps, etc. 120 cycle hum persisted but just on the high-pass. Capacitive filtering just killed the highs that I love so much. Ended up with a 1:1 isolation transformer on that output only. Now it is dead quiet but I have to find a good place to mount it. I know I didn't find the problem but I got rid of it and that's all I care about. By the way, the transformer is from (blush) Radio Shack. Made for car audio, I think.
Bi-amping ROCKS!