Hum problem

@audiotroy 
1.  Mondial magic isolator already installed. 
2.  Will have to check dimmers but usually lights off. 
3.  Floated ground at wall and at power conditioner. 
4.  Hum is present regardless of whether other components are on or off. 
5.  Will try, thx. 
6.  I/Cs have no impact.

@almarg- @almarg- forgot about that; will swap out 6SN7s one at a time. Thx. 
 Have not tried lifting ground on preamp but the passive is a 2 prong plug so methinks that may not be it. 

Not a bad idea, Jim. I’m a regular customer and he lives in my town. If Al’s idea of swapping out the 6SN7s doesn’t work , i’ll give him a call. 

Thanks everyone for your help.  Alas, no cigar;-)  BTW, hum is not so bad that I cannot listen, but it is audible from 8-10  away during quite parts of songs.

@fineito- Not piling on at all.  I'll try anything I can do easily this weekend and then call my tech and VAC on Monday. 

@blindjim- upstream checks:
    1.  Hum not present when Atma S30 amp in system.
    2.  Hum present with 2 pre-amps (one active and one passive).
    3.  Hum NOT present on immediate turn on.  I hear what sounds like a gradually increasing hum after 30--40 seconds.  The relay clicks open a few seconds later and then the hum is very apparent in the speakers.  It's possible that the first hum is coming from inside the amp.  I assume that the first hum (maybe more of a buzz) is the sound of the soft start relay starting to open but I have absolutely zero knowledge of whether or not that is a reasonable assumption. 
   4.  Hum present with multiple I/Cs.  Have not checked speaker cables but seems very unlikely that the hum would be identical in both channels if it were hum induced in the speaker cables.

@almarg- I swapped in about 12 different 6SN7s in various combos.  No impact on any aspect of the problem, so I THINK I can rule that out.  Please disabuse me of that if I am wrong. 

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. 

@jea48
@almarg-
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. 

Jim- The Hornings are an older model, Perikles Ultimate.  They are spec'd at 96 dB, so I guess I'd call them moderately high efficiency.  I think I've got some shorting plugs here so I'll try that out and report back later this evening. 

@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. 

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. 

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. 

@almarg -
Rotating the feedback switch back and forth reduced the volume of the hum as the feedback was increased and then of course an increase as feedback reduced back to zero.  I believe that this is expected but of course I totally lack the knowledge to understand why (something about changing the impedance maybe...of course I can say those words without knowing what that means, either).  Any harm in trying some good old contact cleaner on the switch?  Would I need to remove the bottom plate on the amp to do that?  If so, will that expose me to danger from high voltages? 

@atmasphere- Thank you for taking the time to think about this and to reply.  It appears that I was a little unclear in describing the problem.  The amp is dead silent (inputs shorted, no other items plugged in to nearby receptacles) until the soft-start relay starts to open.  For a few seconds I get a buzz which I attribute to mechanical noise as the relay opens up.  Almarg confirms that his Ren 70/70 does the same.  When the relay fully opens, the buzz goes away and then there is a hum from the speakers that is constant in volume and does not change over time (or at least not significantly).  When the amp is inserted into the system, the hum remains the same volume from full attenuation, until reaching around 9 or 10 o'clock. 

I tried shutting the amp down and then immediately restarting as you suggested.  The soft-start circuit kicks in, so we are back to square one.  Quiet, a buzz (only as the relay is opening), and then the hum.