It's very hard to diagnose much not having the amp in direct sight.
If this one had, say, a burnt power stage or a similar gross failure, paradoxically it might be easier, as there are just a few possibilities, but yours involves a complex preamp, with lots of switching (by relays and Fets) which change the actual circuit, by enabling certain parts, disabling others, with crisscrossing signal paths.
Worst is that it's not a "real" 2 channel amp but a complex single one.
The proper diagnosing involves injecting signal and tracing it end to end, switching different modes on and off, then finding where signal disappears.
To boot, it works, sort of, but at certain levels it misbehaves.
Worst case, if it works clean, use it as is with a couple pedals.
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