From the manual
EXP. PEDAL Jack – This ¼” input accepts an Expression Pedal with TRS connection. The Expression Pedal takes over the AMOUNT knob so that you can modulate the DSP effects with your foot. To obtain the full range of the AMOUNT knob with the Expression Pedal, set AMOUNT to 12 o’clock. To change the range of the Expression Pedal, try different settings for the AMOUNT knob. Some acceptable Expression Pedals incl
The AMOUNT control as standard is connected to both FV-1 parameters (pins 20 and 21) so adjusts both settings at the same time and when the expression pedal is plugged in that takes control of both these parameters. The mod adds an extra pot to control both parameters set individually with their its own separate pot. The way it is done has removed the expression pedal connection.
The expression pedal could be used with some further mods but you would also have to decide which control you want it to alter as they are both individually altered with their own pot now.
I am afraid I no longer have a Holy Stain or the old pictures of close ups from the PCB to try and check out the traces to modify, but I would be looking at just a slightly different wiring mod to do this.
Ie. look at replacing R18 that was removed and then altering how pins 20-21 connect to each pot. This would probably work best by unsoldering and lifting one of those pins from its pad (depending on which one you want the expression pedal and 'amount' pot to control v's which parameter you want the side pot to control). I Think it would actually be a neater mod this way.
I think this would work.....
1. Undo the 1st added pot mod wiring and replace R18(to pin 20)
2. Lift pin 21 from the pcb pad.
3. wire pin 21 to the centre of the new pot control.
This should now give individual pot control for both parameter and restore the expression pedal control to the 'AMOUNT' parameter.
If anyone spots an obvious error with this please post.