If you're getting sound from the outputs from inserting signal at the power amp inputs, the power amps are probably OK. It's probably good run some tests on the power amps to see if they're giving full swing into no load with only signal from the Main In. Once you get that checked out...
I'd say that the preamps, one or more, or the effects are oscillating and driving the power amps into some condition that eats more than 6A. I would pull out C12 and C25, the capacitors that couple preamp outputs into the power amps after those two JFET source followers. That prevents the preamps from putting any signal into the power amps, and you can then probe the preamps without blowing the rectifier fuses. Er... I think.
In nearly every "fuses blowing" situation, the rule is divide and conquer. Make temporary disconnects to isolate power sections and see which cuts make the fuses stop blowing.