I had this ’70s Maxon Treble Booster somewhere in a drawer … While I like very much Range masters TB, I don’t like this one, it sounds terrible to my ears.
I redrawn the (very simple) schematic :
So replacing the 2.2nF coupling caps by 470nF (or 1uF) will flatten the bandwidth :
Here it is ! Just perfect with my ’66 Mustang !