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it's an Ampeg SVT3-PROOh.
Odd power stage. Many comments about unreliable bias.
But "both channels"? Isn't there just one channel? One input, tube preamp, tone/EQ, that wacky big-volt-amp (CF+WCF, WTF?), cap-coupled to 8 Gates.
http://music-electronics-forum.com/t10203/http://elektrotanya.com/ampeg_svt-3-pro.pdf/download.htmlApparently you should replace the output bias trim-pot. It isn't clear to me how it would fail "smoke and burn", but never trust 5-cent bias pots.
Remove MOSFETs. NO load. The output should servo to zero DC, less than a tenth-volt.
Measure DC from a N Gate to a P Gate. You MUST be able to trim this to less than 8V, and I'd want to have 4V available. The "low current" end of these MOSFETs is about 4V gate-source; if bias is higher the MOSFETs conduct many Amps and fry. Going lower will give rough sound on small outputs (this beast is not made to play small) but is where you want to be for now.
These MOSFETs are $5 parts but are going out of style. DigiKey has stock of 240 but not 9240. Octopart shows a few. Makes you wonder if you should stock-up while you can, or bail-out ASAP.
For initial tests, load just the two sockets with 330 ohm resistors to the "protection" transistors. This should do big signals clean in no-load or in 50 ohms (or 100 ohm 50W in series with a speaker). If not, you are only down $10 instead of $80.
The bias process is not in the info I found. If run "cool" then small (<2V) signals will kink through crossover. If too "warm" it will play great but with some danger of thermal runaway and disaster.