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
@almarg-
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. 
@swampwalker,

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.

Edit:
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.

Jim
Jim-
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. 
BTW, RFI was most likely from powered wireless connection to internet for my music server
Did you kill that thing when you were isolating things to find out what might be causing the hum?

Has this amp always had the hum issue? If not, can you think of anything you may have changed or added that maybe now is causing the hum?

I would rule out what you mentioned below before putting my amp in the mail.  Someone else chime in as I also have zero technical knowledge.  But all my troubles have always been due to tubes being bad/going bad. 

To all- I will try swapping out 300bs but it's also hard for me to believe that all 4 were identically bad, resulting in identical hum in both channels. Again, if I'm off base on that, please let me know. As I've said, I have ZERO technical knowledge of how these things operate.