Okay, this is my finding, based on your feedbacks and comments:
1. I first turned on the power switch of almost new Crown DC300A for LF, and heard just a little audible hum from the speakers.
The hum was a bit louder when I turned the volume up to its maximum.
2. Next I turned on almost new Crown IC150A for M/HF, and heard almost the same amount of audible hum as DC300A.
The hum was a bit louder when I turned the volume up to its maximum.
3. The hum was overall just a little more audible when I turned up the volumes of DC300A and DC150A all the way down to their maximum.
4. Now comes this interesting discovery.
I turn on the vintage preamp JBL SG520, with the active crossover Ashly XR1001.
The hum grew more audible when I slowly turned up the volumes of both power amps, but with the preamp volume at its maximum low.
When I turned up the preamp volume, I didn't hear any change in the audible level of the hum, though.
So, do you think this audible hum is more or less related with this vintage preamp?
Thanks much for your diagnostics in advance.