Either one will work for the volume pot. C17 and the pot do form a high pass filter, the A50K will move the lowest frequency from 28Hz to 14hz, not much of a change and these frequencies are below the audible range. Also the A50K will give you the same feel on the control because of the taper. The B25K will have the same cut low frequency, but will alter the feel of the control. There are some pedals that use the A taper pots and some use the B taper pots for volume, I can't tell the difference to the sound or feel. So either should work here.