First suggestion. An amp like this didn;t fail because it was mis-designed. Altering the circuits and changing things adds more ways for the unit to elude repair. Always - in my opinion - make the amp work first, THEN do any modifications.
This amp is pretty much the 400BH power amp board, (used in MANY Peavey amps) by itself with enough stuff thrown in to make it a stand alone power amp. Solid reliable circuit.
Just in case, you do know that of the 8 power transistors, the end ones, Q11 Q15, are wired as drivers, even though they are all the same type. That makes Q5 Q10 pre-drivers.
If R43 burned, it is almost certain Q5 was bad as well. And as soon as I see that, I instantly suspect Q10, R51, plus R49,50. That while vertical stripe through the schematic. If you are replacing the whole output section, then no time to be cheap on the preadrivers for the couple bucks they cost.
You replaced all the outputs? 8 or 6? Usually just a couple fail, one per PS rail, but all is OK. WHat did you replace them with? Please have them all be the same part type. The schematic wants 6357. That part has a newer part number of 70484140. Those are interchangable. The generic for those house numbers is MJ15003. You can mix those as well. I hope we didn;t go to NTE parts. Never mix them. All 8 NTE is better than a couple NTE mixed in with Motorola. Of course NTE will cost mutliple times what MJ15003 cost.
The variac is a powerful tool, I keep mine next to me, but just as important as the variac is the current meter watching the current draw. The point of the variac is not so much the slow rise in voltage, rather we want to slowly bring it up WHILE WATCHING mains current draw. If the unit current starts to ramp up, back off immediately. DO not keep turning it up until fuses blow, that defeats the purpose. You need to watch current and back off before something burns.
No, do not operate the amp with P7 P8 disconnected. The circuit expects the current path through the outputs to be there. WIthout it, ther is nothing but those little 47 ohm resistors to complete the circuit. If R51 burnt, then expect Q10 to be bad
I buy MJ15003 from Allied or Mouser. But PV prices are not outrageous. The little 5331 and 5332 predrivers I always just buy from PV. You can sub them, but the real ones are not expensive, and the wire leads come preformed to fit the boards.
If enough current went through R43 and R51 to burn them, I'd assume Q5 and Q10 were stresed in the process. SOrry.
MY approach for this amp, and the 400BH and similar it resembles, would be to check each power transistor for shorts, verify all the cement power resistors as not open. Don't forget the R52, R53 in the middle there. Check Q5 Q10 and all the resistors in that vertical circuit. Work without a speaker load until we have the amp stable. I'd verify the extremely important dual diode that P6 connects from the power board to the main board. That is CR12. It should test like two diodes in series - about a volt drop instead of half a volt. Also make sure CR13 with 33 ohm resistor in parallel. If any of that diode-ness is open, then both polarities of output will turn on hard at once, blowing fuses. Above them are a couple power resistors 2k 5w R36, R37. Either open?
SOmething I used to do, when working on a blown up amp, was make a work copy of the schematic. Never draw on your real schematic, if you need to scribble, make a copy for that purpose. If I find a burnt part, I use a yellow marker and highlight the part. IN a well blown amp, this can lead to a number of parts colored in, and it reveals a failure current path. And that can suggest parts to check or replace. For example If R43 and R51 burn up and Q5 is shorted, coloring them in leaves a gap in our yellow line - Q10. SO we suspect Q10 in that case.
Start with that stuff and see where we get.