Need help with Silverface Champ mod

[sshan2525]

I am not very technical so I hope you'll bear with me. My friend did the tone stack bypass mod on my ’75 Silverface Champ as shown in the schematic below. It works great. Wen engaged, the amp has tons of extra volume and gain. The trouble is, when not engaged, the treble control doesn’t work properly. It seems to work normally when turned up but adds distortion as it is turned down. The bass control seems to be ok. Any ideas as to what’s wrong? Thanks for the advice.

[Jazz P Bass]

Did the amplifier function correctly before the mod?
If so, something was done wrong to the original circuit.
You have the schematic.
Time to trace it out.