McIntosh C26 - 1st to totally stump me in 37 years

I suppose there are other technicians on this forum. I am at a roadblock here. I have my own C26. It had an intermittent low level "groan" on one channel. It had never been re-capped, so I went for it. I used ONLY Nichicon 105C long life caps. All caps are the same or higher capacitance. All are a higher voltage rating. Now I am left with a 120 Hz hum. Not horribly loud, but loud enough that the preamp is not useable. All four main electrolytics were substituted with other ones to no avail. I left in the new ones. ALL capacitors in the ENTIRE UNIT have been changed! I have spend DAYS on this one item. It is coming close to the "throw it in the trash" point soon. ALL components on the power supply pcb have been replaced with new 5% resistors and Nichicon caps. If I remove the lead from pin 2 on each side pcb and inject signal from a portable audio generator or an ipod, everything is NOISE FREE! When the source items are turned off and volume set at MAX on the C26 all is silent! ( I am feeding my sources to the C26 through .05 uF caps for isolation.) I am stumped to the max. This problem is probably too involved for a forum, but I am taking the chance someone might have an idea for me.