When you say that the hum is present with the amp not connected to the preamp, are you shorting the input to the amp or just leaving it with nothing plugged into it?
What is the difference between a 60HZ hum and a “pseudo 60HZ” hum?
My sense is that it is one, or more, of the power supply caps in the offending amp. Often there is no visual indication that the caps have gone wrong, but sometimes they bulge, or leak. Have you inspected them for irregularities? If you can measure the leakage in the caps that would be useful, otherwise my sense is that a trip to the stereo doctor is in order. I would look at the cathode bypass caps while I was at it.