R18 in combination with R19 form a voltage divider that is the same as having a volume control set at a fixed level...a standard 10% audio pot would be just below the half-way setting (11 O'clock). BUT....more to the point is that the R19 100k has a 500pF cap parallel to it, which, coming after the 1M R18, will be bleeding all of your high end away. The volume shift of the two resistors might well be needed to keep the P.I. from distorting to much...but one could just try replacing the whole R18, R19 and C10 combination with a 1M pot as a pre P.I. master volume.