Input voltage and your poweramp

"Normal" at the Mains should be 125 to 130V average. You could have some voltage drop at your listening room outlets, depending on what else might be running off the same phase(high current draw appliances, HVAC, etc) at the time you measure, and the length/gauge of the wire leading to it from the breaker box. Mine measures 125V(mains and room), when the AC isn't running.

I don't think your problem has anything to do with voltage. I believe your heat sinks are screwed to the chassis too tightly and so they are ringing (think tuning fork). This wouldn't be a Krell, would it? I know some Krells do that. The best way to stop it IMO is to BARELY loosen the heat sink mounting screws so the system becomes more mechanically lossy. Something like 1/8th turn or less. I helped an exasperated guy here on Agon solve this problem that way and the ringing never came back.

Ping, incorrect voltage, either high or low is hard on poor quality transformers. 116v hardly qualifies as low. 130v borders too high. Push the tech harder. I hope you are under warranty.

They're GamuT M250 monoblocks. I have 2 pole main coming into the house. Circuits on 1 pole measures 116-118V. The other pole measures 121V. Moving the amps to 121V sockets didn't make any difference. Also tried an UPS with voltage regulation. GamuT is suppose to be sending new transformers. The question is will the transformers fix the problem?

I'm betting yes. Post your results please.