Yes, my mistake. Meant R21.R20? Don't you mean R21? C19 & C20 are already so large that making them bigger won't change the sound. C12&R16 roll-off below 160Hz, you moved it down to 16Hz. C16&R20 also roll-off below 160Hz, but since part of the DRIVE pot is in series with R20, the 160Hz roll-off moves down in freq when you turn DRIVE CCW. R18, R19,C14&C15 create a gradual roll-off below 1.1KHz.