Ben,
I’ve never done this mod, but theoretically it shouldn’t alter the sound. The gain will be different, so you’ll have to adjust the gain trim pot again by ear. But it shouldn’t affect the bandwidth or frequency response. The signal through the optocouplers will be going into a higher impedance (less load), so the response of the LDRs may be slightly different, which you could adjust with the tone trim pots.
-Brach