While it might change the frequency roll off a bit, I doubt it'd be noticeable. But, the 100n is basically already letting all the worthwhile frequencies through
Given the signal path is so simple, there's not a whole of places to really change much, unless you want to tack solder components to the board.
EDIT- increasing R4 would also be beneficial to rolling off some highs. Could try a 100k pot (wired as a variable resistor) in that spot, and find the sound you like most, then solder in the closest value resistor you have on hand. If you go this route, I'd recommend leaving R3 at 2.2M