I want to mod my fender aa864 to add a master volume ( post phase inverter ). But I have a question... The cable thick does matters? The cable that I have is a XLR mic cable.
I assume you using the PPMV circuit on the internet. No cable thickness does not matter in a post phase / pre power tube position. Mic cable should work fine.
I would not recommend that. It will break the feedback of the 220 from the speaker to the 100 on the PI. It will sound completely different. The feedback flattens the overall frequency response. The only way to tame that beast is with a dummy load.