Preamp/interconnects cause amp problems?

Just because the dealer could not find anything does not mean there is no problem. case in point, I bought a used preamp which had been at the mfg's service center for an issue which was supposedly fixed. The seller had it shipped directly to me. When I got it and hooked it up, the right channel quit working. Changing tubes didn't help. When I called the seller, he was shocked and said that it was the same issue he had sent it for repair. However when talking with the mfg, she too had left it running for a week trying to ;locate the issue and found nothing. I sent it back to her and declined the sale. Anyway, I learned later from the seller that it was a resister gone bad.

Some things happen like that and when your car has an issue that never shows when you take it to the mechanic. Life, I guess. The frustrating part

What model amp is this? Was everything OFF, not standby, that is connected to the amp when popping started? When you turn the amp off and then on again, has the preamp been on the whole time?

Preamp was off. 100% off. DAC and phonostage are always on 24/7.

when I turned it off preamp
Is always connected. Tried with nothing connected and also with a passive connected and same thing .

Sounds like for sure the power amp. The best way to troubleshoot problems like this is with freeze spray. Your tech should be aware of this technique. I have found failing pre-driver or driver transistors were the cause in some amps.

A bad resistor in the power supply can cause the popping sound you mentioned hearing at night some amps will do this when there are issues in the power supply. If the manufacture would of had speakers hooked up to the amp they would have heard this the first time it was on their burn-in rack, they also should listen and not just take measurements. 

This time around i'm sure they took their time and fingers crossed all should be good.