Here's a larger schematic:
http://www.experimentalistsanonymous.co ... %20EM5.pngThe mix control is VR1, and the reason it can only do 50/50 is that it's just a volume control on the repeats. The clean signal is always at full volume. SO! There are a couple of ways to go about this.
The clean signal connects to the output buffer through R19. You can add a volume control for the clean there.
Or, you can bypass the mix control that's in it, and use it to blend between the dry signal and the repeats. I imagine the pot is board mounted, so that's going to be a bitch. But so is adding another pot, soooo
Here's the easy way, but it's not variable. You can just lift one leg of R19, and solder a wire from that leg to a SPST toggle. Then solder a wire from the hole where that leg used to be to the other lug of the switch. That'll turn the dry signal off.