I think this might be a "scratch ok" control. To my untrained eye, it looks like it's interacting with the bias control, as a headroom control, which implies a change in voltage. I finished my build last night and have the same result, but it doesn't bother me. I can only imagine having to dial that in when the pedal changes chains or instruments, give it a tweak, make sure it's happy, and forget about it. If it's really a problem, pick some low and high values in the pot range (25/75%) and turn it into a switch, as I've seen mention of regarding other designs. Or, set it up for the most headroom with a fixed resistor, and then fill in the hole!
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