+1Tonestar1 said:They haven't wisely tested anything. All they have done is plug it in and been unable to recreate the issue.
A shop sales person plugging it in for a few hours is not testing it. Unless it goes back to Yamaha to be properly checked or at the very least a suitably qualified and experienced electronics engineer it has not been properly tested.
This is why you should not accept the same amplifier back again and should demand either a brand new replacement or a refund.