Quad 909 heat and hum

I am very happy with my Quad set-up. I recently came into some issues that I discussed with Quad USA, but I want to get the opinion of other owners out there.
I have it hooked to a Quad 99 pre-amp with the supplied Ampbus or with regular interconnects, and into a pair of Usher CP 6311's via Audioquest Slate cables, all plugged into a PS Audio Duet with the stock cords. Using a Sony 705ES CDP for now.
First thing I noticed is that when in stand-by (stand-by mode of the preamp, led on the amp goes red, everything goes quiet), it continues to get very warm. I could have sworn it was not this way from the beginning, but Quad technical team insists that ought to be the case, and the standby mode is nothing but a fancy muting button. Anyone else noticed how warm it is in this mode?
I wouldn't have gone looking for trouble, but what prompted this was an annoying humming noice into the tweeter in this stand-by mode. It is not there with a different amp into the same cables and speakers. Swapping the power cord with the better one that came with the pre-amp did not change it one bit. With the pre-amp off completely and disconnected it is still there. Quad's explanation was that the amp is more sensitive than others to interference from the power grid etc. Anyone else have this problem? How do you deal with it?
Should I be worried about downstream components if something was indeed wrong? For now I have it connected with regular interconnects, and fully on all the time (Quad USA distributor advice again).
Thanks a bunch.