Hum problem


Been chasing a hum in my VAC Ren 30/30 for quite a while.  My technician has been all through it and does not find a problem.  Says he cannot hear it in his system, but it's quite apparent on my Horning speakers (94 dB maybe?).  As soon as the soft start relay starts to open, it begins to be audible and when it clicks open it's fully audible, maybe from 5-6' away, with the pre-amp fully attenuated.  Once you advance the attenuator past about 9 or 10 o'clock, it starts to get louder, but not before.  It's not a transformer mechanical hum; no sound at the amp but clearly audible through the mid range of the speakers.  Present w no other components turned on (or any/all turned on).  No change after swapping out power cords, lifting ground, swapping interconnects.  Changing the position of the ground switch on the amp has no impact.  Same w AC straight from the wall or w AC from a Dodd Audio Balanced Power System iso transformer.  Since this is a transformer/tube amp (not an OTL), I assume there can be no DC offset, and cannot really check that because I don't think I can operate it w/o a speaker load and the info I find on the web says it must be checked w/o a load.  

Any ideas before I ship this 85 lb beast back to VAC?

128x128swampwalker
I had this with an Audible Illusions modulus 3a pre. The tech fellow I hired could find no explanation. I used a DAC direct to power amp for about 3 Mos. One day I tried the preamp and the hum was gone. My neighbor was big into ham radio. He had moved. So rf? 
Couple of tips:
1) Try using only 1 outlet for all of your components. Obviously using a multiple outlet strip that can accommodate everything and is rated at 20 amps. Insure that the outlet is dedicated if you can.
2) Experiment floating the grounds on the power amp. If no difference, then try the preamplifier.
3) Ultimately it may come down to new shielded Interconnects.
Best of luck!

Addendum to my previous post above.
**Important consideration when using a cheater plug to float the ground on any component.
Many cheater plugs are polarized. You must eliminate the polarized lugs so you can change the orientation of the plug. One way may not change the hum but rotating it (which you would not be able to do prior to eliminating the polarized lugs) may do the trick.
Then using a volt/ohm meter with everything plugged in but off. Measure from the ground lug on the preamplifier to the chassis ground on the amplifier. Obtain the lowest resistance reading by changing the power plug orientation.