You can add some series resistance between the center pins of that louder pair of RCA inputs and the selector switch. That will be a pad in series with the 100k pot. I tend to do this by experimentation, and I would recommend trying resistors in multiples of 75k. (75k, 150k, 225k, etc.) All you have to do is remove the jumpers from the center pin of the RCA jacks to the selector switch and replace the wire with the appropriate resistors. If they need to be longer, you can use a little buss wire or stripped Teflon wire to extend the leads a bit.
