I am afraid you will have to do a bit of work at component level. Any fault in the gate section (eg an open resistor), can blow the FETs. If any of the FETs blow, the rest will blow because of the load, often almost immediately.
Check the gate drive, that is the section that prevents the FETs from blowing. If the 75339 keep blowing it is usually something wrong in gate.
Double check the resistors, ensure that they are factory specs.
Do not check only impedance, but also tolerance.
There should be capacitors near the FET, check them out to see if they look fine.
Check the zener diode connected the gate. That will cut off if load is too big. If a diode is shorted, then zener will blow.
Generally inspect the board for cold joints.
Check the PSU section.
I do not have precise schematics for this model. If you need to know the value of components (eg. the different gate resistors before FET), then the best thing to do is contacting MTX and getting the the component specs .See:MTX Audio.