When you are measuring the heaters, measure across, not from either side to ground.

When the amp was cutting out, was it instantly or did it take a second or 2 to fade out?

Connect the send to the return jacks with a cable.  Does the problem still happen?

  Try putting a patch cord from send to return.  Is it noisier than without the patch cord in the loop?

All user adjustments are on the outside of the amp.
Never adjust internal adjustments.  They do not go "out of adjustment".  They are there to make adjustments after certain specific parts are replaced.

If your amp has a problem, state what that problem is.  It will NOT be due to an adjuster going "out of whack".   

What is the problem with your amp?

Your description of what the gain pot is doing sounds kind of odd.  Is it possible it is defective and going straight from zero to full?

  Sometimes just giving those jacks some exercise will fix the problem.  Plug your cable in and out of those jacks a dozen times or so, see if that helps.
Usually you also need to spray some contact cleaner in them (something like deoxit or nu-trol) as well as exercising them to work the cleaner in.
  The other option is to just leave a short patch cable in there.

This is the article on negative feedback with op amps:
http://www.allaboutcircuits.com/vol_3/chpt_8/4.html
I don't think it appears on the mouser website although they may have a link to it.

  Thanks JM, you're right, the goop in my "parts cleaner" pail is probably mostly oils now and very little kerosene.  It may not evaporate as fast as some other fuels, but that doesn't mean it doesn't evaporate.  I guess the key is to wipe off as much as you can and the rest should evaporate.

For byproduct I think he means the preamp IC rails after the 1K droppers.  In the stock Lead 12, main rails are +/-19, IC's +/-16V.
  So with 24V rather than 28V transformer, main rails approx. +/-17, IC's +/-14.
Am I close Dr.Gonz?

  I would not recommend plugging the 100V amp into 120V.  If any of the caps operate near their voltage rating, the extra voltage may be too much for them.
If you could use a variac and check them, you may get away with it, but the amp will now be capable of producing more power which could strain the transformer.
As far as the transformer swap, the lead 12 runs on +/-19V, where the MG10 runs on +/-15V, so  the transformers are not the same.

  What a coincidence, I was just breaking out the alcohol too!   
Looks like restaurant/bar grease/nicotine type residue, probably was in the same place for a long time.
  Dr.Gonz:  as far as the kerosene goes, I don't think it evaporates so I wouldn't recommend it.  I filled a pail with some and forgot a motorcycle chain in there about 5 years ago, no lid and the pail is still full.

 There is some speculation it may be similar to Vox Escort 30 due to the Dallas Music connection.
Does the schematic match up?

 Have you tried cleaning and resoldering the mid pot yet?  If so, what was the result?

  Any idea what the resistor goes to?  Looks like some regulators on that board?  Also some jacks?  You've probably checked for a simple problem, but this schematic may be close:
http://music-electronics-forum.com/attachments/23732d1370622849-spider-iii-1508-3012-hd75.pdf
  I would think most of those amps blow the power amp and get gutted, so the boards could be good, on the other hand it would depend on whether the seller was parting out amps or came across a pile of junked defective boards, in which case they could ALL be bad.

  Do you have experience (and tools/test equipment) fixing amplifiers?
Schematic is available here:  http://elektrotanya.com/hughes_and_kettner_matrix_100_amplifier.pdf/download.html