Need HELP-Troubleshooting weird balance and hiss issue on Parasound 2100 preamplifier

Ok
Bear with me on this weird issue.  Last year I forgot to turn off my amp as I switched out my left speaker and  I crossed positive and negative wires. After the loud pop sound the only thing I could determine as far as damage seemed to be, as impossible as it sounds, on the preamp. Now every time I first turn on my preamp I would get a loud hiss that would slowly die out after 15 seconds. The left channel also seems to have a significant drop on output. ( It follows the channel as I can move it to the right speaker by swapping interconnects). Switching speakers -amps-cables ect...does nothing but show its from the preamp -I assume its output stage. Local service bench tested it out and could not replicate hiss. For the out of balance issue I can apply pressure downward on the balance knob and will even out the right and left channels. This is crazy but I actually have temporary piece of plastic shoved in to the slot to keep it even. Does any of this story sound familiar and does my temporary fix make any sense? Can