The schematic does not show the PA jack tip connected to R1.   The tip is pin 2, which when plugged connects through auxiliary contacts 5 and 6 down to R99, which is the power amp input.   R1 does not connect to ground, it runs through the other auxiliary contacts and then back over to the input jacks.  It is part of the mute circuit.

Disagreements are not offenses  This is a forum, I stated my thoughts and not everyone agrees with me.  That is fine, and that is how we learn things.

2mv hum is not a lot

The JFET is across the signal path.  I just want to know what happens when you short from r59/C33 to ground.  NO need to short the gate.   While the JFET itself can be noisy, I was thinking your noise is either coming into the power amp through that path or it is after that point

I meant the volume on the guitar itself, we were bypassing all the preamp.  I will assume you did that.

That puts the hum in the power amp.  CAn you scope or otherwise measure the ripple on the 40v rails? 

Q7 is the mute JFET at the input of the power amp.  SHort across it, does that kill the hum?  Or remove Q7.  Same question.

If you want to earth it, then earth it.  Peavey made any number of models in the modern era with the two wire power plug, and met whatever electrical approvals they are asked to.  What those are I cannot say.  That was all I was thinking.

There is Deluxe 85 and Studio 85, but there is also a separate model that is simply "85".  I have it on paper in my stored files - not really accessible.  I have been meaning to dig out some of the old paper files , but no idea just when.  But I do have an inventory of them and it is in there.

If TP 3,4,5,6 are right - and they are - then TP1,2 pretty much had to be.

OK, so no DC on output and rails are right.   You might look for ripple on the supplies, but looks like that isn't the deal.

Plug the guitar into the POWER AMP IN jack and dial it to zero.  Hum still?

OK, if there is no DC, then my next suspects are the main filter caps.  The two standing side by side near the heat sink.  I tend to think they are OK, but may have cracked their solder.

The main rails are 40v each,  are both up to that?  Or is one substantially lower, like 25 or something?  That would indicate large ripple and lack of filtration.  Or certainly you can meter or scope for ripple.

SO the amp works with no speaker?  At least not blowing fuses?  Measure for DC voltage on the output.  Or look at the speaker cone when it makes the loud hum.  Does the cone move one direction and stay there?  That is DC on a speaker.  It will damage a speaker so no more than a couple seconds please.

DC on the output with no load will not draw large excess current.  Put a speaker or load on it and it will.

It is like an electrical appliance in your home.  If double insulated, no need for earth.   Just my opinion.

I stand corrected.

#1  Y'know, I wouldn't bother sticking a three wire power cord on it.  You describe the original Blazer  It has op amps in the preamp.   If I recall, this had the little open frame reverb guts.  Just the small spring assembly, no pan around it.  It would be the equivalent of an EB type pan though.

#2  The whack test tells you it is a connection.  Use your wiles to isolate the sensitive area.  It may not be solder.  A poor contact in the headphones jack , the ones that are speaker cutouts.  And there are the channels switches, could have dirty contacts.

I just don't think they are there as an effect.  Just don't want to overdrive the reverb or whatever.  The power amp can only put out what the power supply rails allow.  Drive it too hard and it just clips.  I do agree with insufficient heatsinking.

Lift them to see what difference it makes, if any.   No point in designing a circuit modification if it doesn't do anything.   Find out.

Maybe a little experimentation in the R9, R16, R5 area might even up your levels.

They limit the signal by not letting it get larger than the LED drop.  They don't act as an attenuator.  Like your car seatbelt limits you to not going through the windshield in a crash.  The only time they have an effect is on peaks.  They will lop off peaks when they get too tall.