Graaf GM20 and high sensitivity speakers - Hum?

My bother purchased one of these for me in the UK to use with my Avantgarde Trios. Sensitivity for the Onos are 100db/watt and the trios are 109 db/Watt.
The GM20 has been serviced and tried on his Avantgarde Unos. He found that with no music playing he can hear Power supply hum from his listerning position 9 feet away bearly audioable but makes the listener aware they are hearing a stereo system. The interconnects and layout are not the source of the hum. This is not noticable when music is playing at normal levels but is in silent passages. He contacted the service center who state that it is 80 db below the noise level and this for an OTL amplifier.
So is this normal and does anyone else out there use a GM20 with high sensitivity speakers without having this hum present?

I would hate to make him carry it overseas to Los Angeles if the hum can not be removed.

Presently my system runs in balance mode consisting of a Mark Levinson CD390s, Pass Lab Aleph J or F3 dpending on mood though Kimber 8tc cable.
Any information would be greatly appreciated.
Russell B