So the N64 was not too bright? It seems the brightness of your SNES is far too high. Are the RGB wires from the amp to the multi-out pins connected through 75 Ohm resistors? Can you run the 240p suite grey ramp test?
You might also need to attenuate the RGB levels. I have a PAL 1-CHIP-02 that needed 750 Ohm resistors to ground on the RGB lines before it would show proper brightness on the OSSC. My Sony TV didn’t have a problem with the higher brightness when the SNES was directly connected but the OSSC is very strict about video levels.