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?

And is there any way the speaker cables could be causing the hum? Capacitance? Inductance? I have read where some speaker cable designs can cause problems for a power amp.
Thanks for your comment, Jim. You may be thinking of the fact that a few speaker cable types having ultra-high capacitance have been known to cause some amplifiers to oscillate, with destructive results in some cases. Goertz and the old Polk Cobra cables, for example, at least if they are used without a Zobel network connected at the output of the amp. The reason for the ultra-high capacitance in those cases being that it is a side-effect of their approach to achieving ultra-low inductance.

Such an oscillation would most likely be at a much higher frequency than is the case here, though, and would typically occur with an amp having low output impedance, considerable amounts of feedback, and particularly wide bandwidth. None of those characteristics apply to Michael’s VAC Renaissance 30/30, and in general are most likely to occur with solid state amps.

Best regards,
-- Al

My speaker cables are 8' Jade Audio Vermeil.  I have no idea if they have have ultra-capacitance or ultra-low inductance.  In fact, I have no idea what inductance or capacitance are, beyond knowing that they are electrical characteristics or parameters.  JD's web site does not have any specs. 
Hum still present when amp turned on w shorting plugs in input RCAs.  With nothing connected to the inputs (not even shorting plugs), I get a loud buzz; much louder than hum but I believe it "on top" of the hum. 

By chance did you unplug all the other associated audio equipment from the wall AC power outlet? Audio equipment near the amp as well as the speakers?

By the way the reason you hear a loud buzz from the amp/speakers with the shorting plugs removed from the inputs is due to RFI/EMI that is allowed to enter the open input ports/jacks of the amp.

By chance do you have another pair of speaker cables you can try? If you still have the hum using different speaker cables that would rule out the cables, ime.

Also are you using an aftermarket power cord on the amp? If so did you take the power cord with you when you took the amp to the repair shop to have it looked at?
Just for the heck of it install the factory power cord that came with the amp. Check for the hum.

Thanks for your continued interest,
   1.  Unplugged all other equipment in vicinity from AC.  Amp w shorting plugs only... still hums.
   2.  Then changed speaker cables...still hums.
   3.  Then changed aftermarket cord to a heavy duty, OEM-type AC cord w molded on male and female connectors  (no idea if I even got an AC cord when I bought amp used)...still hums. 

Jim/Al- I'm guessing that we have now isolated this problem to the amp, no?

BTW, RFI was most likely from powered wireless connection to internet for my music server.  No change in hum pretty much rules out the server and ancillaries, I imagine.