We get more new members this way! Welcome to OCCD land.
In the M-1.0, rule number one is: Do not under any circumstances jumper across the triac (TR 401). You will destroy the whole thing.
Rule number two: In this amp, a problem in one channel often shuts down both, so don't automatically suspect just the power supply.
Now, to answer your main question, Ohm-check (Rx1 scale) all the power transistors, 10 on each end, for shorts. If I needed to tell you to have the unit unplugged to do this, you probably should send it to Oregon for repair. Check each of the three leads to the other two, both ways (i.e. reverse the ohmeter leads and do it again. That's 12 tests per transistor!) Half-scale readings are normal, near-shorts are not. Report results and then we go on from there. If there is a question, the other channel (other end) should read about the same; a big difference indicates a problem.
Also, for possible later use, determine the version of M-1.0 you have. Is there a switch on the back for mono use? That's the "non-inverting" type. No switch= "inverting" type - slightly different schematic. Note: the switchless types can still be wired for mono use.