Just reading above, it sounds like Latedev is speaking of Q13 as if it is a driver, Enzo speaking of it as if it is an output transistor.
After looking at the schematic you supplied, Q13 is an output device.
Latedev, are you looking at a different schematic?  A shorted Q13 could be C-E, so it would not necessarily affect the bias.
Operating with one pair of the output transistors missing is not one less gain stage, it is less current gain.  This translates to less power available to the load, and more strain on the other output transistors at higher power levels.

  If biasing at that level has taken care of the red-plating, then you are probably ok.
But I would caution that you may have a higher dissipation (%) than you think. 
Old production 6550's were rated for 42W, some new production ones are only rated for 35W, depending on brand.  Check the specs for the brand you are using.

  From your description, the red-plating is not occurring at idle, but only when pushed hard into load, (assuming full power or clipping).
  In these cases, it can often be cured by decreasing idle current.  I agree with Enzo, is is still idling a bit hot and could be reduced.
  Idle current can often be reduced below 60% with no negative impact on tone.  Many amps are biased around 50%, so I usually suggest biasing as cold as possible without having a negative impact on tone.  This will increase tube life.
  Another reason amps sometimes red-plate at high volumes is load mismatch, where the load impedance is lower than what the power tubes would like to see.
  What load impedance is the amp supposed to have, and what are you using for speakers?

  The control grid stoppers are probably not a bad idea.  Marshall's with 6550's used 1K5's, same as Fender uses in most tube amps.

LateDev:  why change the 1K screen R's?  That is the stock value for this amp with the stock 6550's, also used by Marshall's with 6550's.
The EL34's in this amp weren't stock, the schematic is for 6550's.
And which resistors did you mean to lower to 47K ?

Quote from: LateDev on June 30, 2015, 04:00:09 PM
I guess I will just have to be disappointed and you will have to carry on in the mistaken belief it is a test device.

I realize it is not intended to be a test device.  But when something acts as such on occasion, incidentally, why not use it to your advantage.
Consider the example I was referring to.  The bulb lit up with the rectifier tube in place, but did not when the rectifier tube was removed.  Had he used fuses, the conclusion would have been the same:  fuse blows with rect. tube in, does not blow with rect. tube removed.  By logic we conclude that the fault is after the power transformer.
The bulb limiter has served an unintended function, and has in fact been part of a test.
Yes, as Enzo pointed out, it has limitations.  But that is not to say it can't serve more than one function.  A fuse can serve as a "go/no-go" current measurement, in the case above, the bulb did the same.
  Your unequivocal statement was too sweeping and misleading in my view.  Had it not been absolute, I would not have been so opposed to it.
  We must make the most of the tools we have at hand, and sometimes that means using them in ways they may not have been intended for.
  I don't believe the bulb limiter is a test device, but my belief can not prevent it from occasionally being so.

Quote from: LateDev on June 30, 2015, 11:23:21 AM
You should never use the bulb method as part of any test

That is just plain wrong.
As Enzo mentioned, the bulb in this case clearly shows the PT is not the short in question.
The bulb limiter is used extensively for rough fault finding on this forum.  If you are that opposed to it, you're gonna be real disappointed around here  .

Ok, we are just misunderstanding terminology.
Front end of a power amp is the differential input or whatever circuit is at the input in the power amp itself.
Preamp is preamp.
At least that's how I was always taught.
When you said "front end" in reference to the preamp, I thought you meant the input stage of the power amp.

Quote from: LateDev on June 26, 2015, 06:58:39 PM
I just wonder why you want to use a Behringer circuit for the front end to a power amp ?

Who said that? 
Teemu mentioned using it for power amp emulation in the "later stages" by which I think he meant later stages of the preamp.

Yes, bad connection to the socket could cause squealing.
The power tube heaters are all in series, so you need to verify all the socket solder joints, and verify the socket pins are contacting the tube pins.
Your description of the tube holder being loose sounds like the socket pins need to be retensioned.

If the volume is up, even with a grounded input you will get hum waving your hand around the input area of an open chassis.

  That waveform does look like something a bad cap could cause.  Do you see that kind of waveform on any of the supply rails?

Not sure if you mean little caps, or maybe SMT caps.
I don't often measure little caps, or any other for that matter.  More likely to find bad ones from fault symptoms and in circuit measurements.
But they do have their uses, and having the right tools for the job is very valuable when required.
I have a DE-5000 LCR meter that is well regarded and relatively inexpensive.  But you need to make sure you get the clips, or SMT tweezers if you need them.  I made the mistake of thinking I could use regular probes and ended up having to buy the right ones.  Would have been cheaper to get them all together.

 Instead of lifting C46, ground it's end that connects to Fet3.  Any effect on the hum?
The unrectified AC that is feeding the relay and Fets is for turn on muting.
When you say there is large or nasty hum on supply rails, please provide numbers you have measured.

  In the meantime, you can put a non-electrolytic capacitor in series with your meter probe to block the DC when measuring AC.  Something like .1uf with a sufficient voltage rating for the highest voltage you will measure, I would use 600V if you have it.

  The voltages will read high if the tubes are not installed.