So if it was just connected to line out from the Jamo, I don't see it on the schematics Raff had, but it should be just a low-power signal, from the op-amp section. And even if the Jamo had some input signal, a line level output, shouldn't damage much of anything. Maybe some IC would not like any negative voltage from a line-out.
In the Yamaha the SW-opamps are on page 115, and their mute circuits are on p117. Those mute circuits are run from +/-12V, where as I think the op-amps in the Jamo are +/-15V rails. In the Yam I see the caps around there are 50V, but they are electrolyics, and could get reverse biased, and fry. Or can some base junction in the muting circuit see too much voltage ?
As for the Jamo, well I wish I had it here, I'd start with it unplugged, and probably find a few problems right away just with a DMM.