I took a look at the manual for the amp, which I found at hifiengine.com. I see that when its "normal" or "inverted" RCA inputs are used RCA shorting plugs should be inserted into the other of those inputs, but RCA shorting plugs should NOT be inserted when the XLR inputs are used. I’m wondering if when ARC worked on the amp they might have put shorting plugs on one of those RCA inputs, and if so if the shorting plugs may still be there. If that is the case they are most likely the cause of the loss of gain you are seeing when using the XLR inputs, as they would be shorting one of the two signals in the balanced pair of signals to ground.
Also, if perchance shorting plugs were in place in the past, when the distortion and power cycling were occurring, the short being placed on one of the two signals in the balanced pair of signals being provided by the preamp could conceivably have been stressing the output circuit of the preamp in a manner that eventually caused it to distort and/or power cycle. And that might even have caused the preamp to output some amount of DC, causing the amp to power cycle.