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?

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.  

One more thing to try, although it may be grasping at straws:  With the amp turned off, rotate the feedback control through all of its positions a few times, to wipe off or at least reduce oxidation that may be on its contacts.

The condition of the various contacts on that switch are one thing that comes to mind that could affect both channels in a similar manner.  And the age of the amp makes their condition somewhat suspect.

Good luck.  Best,
-- Al
Jim- yep I had everything off and double checked after reading your post

Al - will try that but if that’s what it is I am going to strangle my tech. The feedback control was kind of funky in its operation and the main reason I brought it in. The switch operates better know mechanically and he SAID he fixed it and also spent 3-4 hours giving it a complete going over. 

Jetter- I guess I’ll have to dig up those old 300 bs.