These PCBs are muted at power up/down but the use of shunt j-fets type J112. When connected to 2 wire equipment the earth leakage current can damage the FETS through over voltage. If the volume is low and/or the sound is distorted these FETS need replacing. Any if the J prefix switching FETS should be OK. Currently I fit J109 as they give a very deep mute the on resistance being so low. To prevent this fault recurring in the future, fit a bi-directional ESD diode in parallel with each line output. Suitable diodes are P6KE18CA see P6KE18CA Littelfuse | Mouser United Kingdom. The diode must be the bidirectional type. Add an extra 100R resistor in series with the wire to the output connector to limit the current in the diode.
If this modification is a bit too difficult a workaround, is to remove all mains plugs before changing any of the interconnects to the preamp.