In this case I would recommend audio probing a couple of key points. Check pin1 of IC1A. It definitely should not be dark there no matter what the gain pot is set to. Now check lug2 of the Clean portion of the gain pot, then both sides of R11 (in both cases where your "dark" setting is at on the gain pot). You could modify some values in those sections to brighten it up, if you find either area to dull the tone up too much.
Possibly, it just doesn't work well stock with your setup. What kind of guitar and amp are you testing it with?
Possibly, it just doesn't work well stock with your setup. What kind of guitar and amp are you testing it with?

