I've had one and only one incident with mystery tone burst out my G68J

It was during an MRC session. For no reason it went in to what must have been acoustical feedback in all channels

it's not repeatable if it is a bug, it only happens every 4 year, god luck to anyone who tries to debugg this. Anyway i was close by and run to my amps and turned them off, so it saved my speaker drivers.
Another factor was that i have one amp with adjustable gain and the rest hooked upp with rothwell attentuators that takes away -10dB of gain, so my total system gain is pretty low.
/Mikael