The jacks for the effects loop are stereo or something and when you plug into them then its like a serial loop? anyway I tried to bend the jack around so there would be no connection if the jacks were unplugged between the power amp and preamp but I guess i did it wrong because sound would still come through and my effects made my amp sound crappy
do you have a schematic? you just have to unsolder the signal feed from the pot. it's either pin 1 or 3 can't remember which. You could even label and unsolder them all from the pot. Don't forget to insulate! also don't forget to discharge the amp. Jacks r not stereo but at least one is a switching jack.