>>Btw...amp plugged in alone is dead quiet as I suppose it should be as it uses B&O ice power class D.<< Is this statement an error?The more you describe the noise the less
chances of it being A/C power hum or bad power connection is less likely.A lot of power utilities will check for a poor connection in your breaker box as a courtesy.If it is not anything in your gear causing it,it still sounds like something electronic causing the problem,especially different gear doing the same.A fan speed control,even in heat/ac system,microwave oven timer clock,and on and on.A lot of appliances always have something electronic running when you think they are totaly off.Even sprinkler system timers.
chances of it being A/C power hum or bad power connection is less likely.A lot of power utilities will check for a poor connection in your breaker box as a courtesy.If it is not anything in your gear causing it,it still sounds like something electronic causing the problem,especially different gear doing the same.A fan speed control,even in heat/ac system,microwave oven timer clock,and on and on.A lot of appliances always have something electronic running when you think they are totaly off.Even sprinkler system timers.