A few comments.  You mention "power supply rating", but that is the rating of what the amp uses from the wall outlet.  Just ignore that number, there is no way to say how efficient the amp is, only we know it will be less than 100%.  You can't really get any useful info out of the 120V rating as far as power output.
Now, as to the RMS values given for various outputs, it will change depending on the load impedance.  The RMS into the speaker is a function of the DC supplies to the output transistors.  This will change depending on how much current is being delivered to the load.  You won't get the same RMS into a 4 ohm load as you will into an 8 ohm load.  This is because the power supply will "sag" when asked to deliver more current.
  Look at the Bandit for example.  80W@8ohms.  This works out to 25.3VRMS as stated.  But they rate 100W@4ohms.  Working backwards from that, you see this is only 20VRMS.  This is because the power supply sags when asked to deliver more current.
  The RMS value is a function of the DC supplies.  Converting 20VRMS we get roughly 56V peak to peak.  That would be the voltage available to the output transistors from the DC supplies at the time of measurement.  So at full power into 4ohms there would be 56VDC between the + and - supplies, or +/-28VDC.
That is what you would measure at C72 and C87.
When the Bandit is running into the lighter 8ohm load, the supply does not sag as much.  The 25.3VRMS works out to approx. 70.8V peak to peak.  So there would be +/-35.4V at the supply rails at this time.
  I hope this shows how the RMS voltage does not stay constant as the load changes.
  So the Bandit will run higher supply voltages to the 2 transistors, that is why it can deliver more power than the other amp that has the same transistors.  To get the extra supply voltages you need a bigger power transformer.
  To run the transistors at higher current and power, you need better heatsinking to keep them from burning up.

Yes, you already have a send jack, you just need a return with a switching jack so you can run straight in to the power amp if required.
Also, the protection diodes like CR7&8 in the express/studio are a good idea.
In the attached diagram, I've cut the circuit and added the return jack and protection diodes from the express/studio.

** If anyone sees errors or omissions, please speak up!

  If the amp could be killed by a 10Khz signal, then it can be killed by a high note.
Putting a cap in to kill 10khz is going to hurt the treble response of the amp.

  As well as checking the line out as Roly mentioned, you can run a signal into the FX return jack when the problem is present, and see if the return is working.

Oops! Sorry, when I said go with the values in there, I forgot these were not the stock parts.   
The supply schematic attached says they were 4700uF for this model.

  What you are really asking the amp to do is deliver more current.  To accomplish this you would probably have to upgrade the power transformer, maybe other power supply components, and heatsink (or add internal fan).
  It's not really practical, would be much more economical to upgrade to a higher power amp.

  Best to stick with the value that is in there.  If they are running at a voltage too close to the rated voltage, then you should go with a bit higher voltage rating.
And higher temperature rated caps (105C instead of 85C) should last longer.

  Agree with JM.   I was unaware you were wanting to use a multi-speaker cab.
To run all 5, the amp speaker would get half the power, and the 4x10 cab would get half the power.  Not much point in that.

  Your math seems ok.  This amp had 2 x 12" speakers so they were probably 8 ohm wired parallel for 4 ohm load.

  Well, this is the first time I've seen a 6 ohm spec for any type of music gear.  But apparently peavey had a custom 6ohm speaker for this model?
  What speaker is currently in the amp?  Is the weber 8ohm you spoke of in the amp presently or is that what you wanted to add as an external speaker?
  To put it most simply and practically, with a solid state amp if you run it into less than it's minimum rated load, it will overheat and burnout the power amp section.
  Sometimes you will get away with it if you don't play too loud, or add some additional fan cooling.  But other times you will just blow the amp so you really don't want to take that chance.
  The best thing to do would be to wire up the amp so it puts the additional speaker in series with the internal speaker.  Then you will have a little less overall power, but probably not noticeable.  And you will not be in danger of blowing the amp.

 

  It's non-conductive in it's normal state.  But you don't want it on the tracks anyway.  Just where you have mechanical contact of moving parts (aside from the tracks and their contacts).  The tracks you would clean with a regular contact cleaner that contains a lubricant, whatever you are using for regular pots.

  On the borders of the page are numbers (top and bottom) and letters (sides).  It's just a way of describing areas of the schematic.  Sort of like latitude and longitude of  a map.  So find A on the side and follow across to 4 on the bottom and you will have the general area.

  I'll just put both power amp schematics up anyway.  Might come in handy for someone in the future  :cheesy:

  There are a few power amp versions, what are the output transistor types?
Preamp schematic attached.
Deoxit makes a fader lube, as you say it is expensive.  Other types of cleaners tend to change the feel of the faders.  In Canada you can find the deoxit sold by Hosa accessories at music stores.  Alternately you can take the faders apart, use regular deoxit or equivalent, then lube the moving parts with white lithium grease or similar.

  Sorry I wasn't more clear about pins 7&8, using the word "signal" on the data sheet was a bad choice for them, as Roly mentioned, pins 2 & 3 are the inputs.
  The chip has both standby and mute options.  In the Marshall they utilize the standby (there is a delay circuit which acts to mute the chip during power up at sector A4 of schem.pg2).  Because they use the standby, they tie the chip mute to the +supply so chip mute is never activated.