I've just re-read some of your posts. If there was a fault with the PSU on the DAC the hum would not vary by moving the cables, it would be constant.
Since you are only using an optical input it narrows down the problem. You need to find out what the source of interference is. Get the DAC away from any power supply bricks, transformer etc, pull it right out from your equipment if need be. Move it around and the analogue output wire until you can find where it is picking the interference from.
Right i've experimented some more with this.
I've got a lump of roof lead and stuck it next to my DAC (isolated it by so doing) and changed an inerconnect that may have been a contributing factor.
Obviously a lump of lead is going to stop nuclear radiation as well as electrical interference. I've only got so much lead tho!!
For the purposes of testing further; what can i (for example) wrap cables in to stop interference. eg could i run them through a bit of copper tubing or wrap some kitchen tin foil around them?
I've now also noticed some highish volume distortion with video-audio playback, which may or may not be related.
It keeps pointing back to the DAC. But I don't want to fork our >>£100 to get something serviced if it ain't broke.
any more suggestions from you knowledgeable lot gratefully appreciated!