I popped into Marks around lunchtime, pity we didnt catch up to chat about your DE-hum problem.
As Mark mentioned to me, most machines are quietened be mounting the transformer on a rubber or silastic bedding so they dont hum the cabinet.
Since yours seems to be in the speakers, you'll need to improve on the poor electronic design, since replacing caps isn't helping as yet.. 
Marty Machine
I tried Mark's method this arvo. I put about 7mm of silicon around the centre mass and pretty much in a square where the transformer sits. Didn't really help unfortunately. There is some noise (if I put my ear under where it sits, but it doesn't seem to be a vibration type. The G.I noise I will put up with as that is now accepted as norm. The other humm (also noticable mostly from right speaker) I will still need to look at....
