You think something else in the amp could be causing the input selection issue, or something else in the amp could be causing my perceived "low power" issue? Any idea where else I should look besides the selector switch?
Well, yeah, it's possible that something else in there is faulty. You did describe them as "well used" after all. I hope not, for your sake, but anything is possible. Cleaning the input switch with deoxit is a good idea, low hanging fruit and all that. But unless you really know what you're doing, I suggest you don't start probing around in there.
The input sensitivity difference is responsible for less percieved power. You said the Classe needs only 1.37v (seems more like an unbalanced input sensitivity?? Does the classe specify balanced input sensitivity?) to hit rated power, while the Mac needs 4.2v on it's balanced inputs, 2.1v unbalanced. (I am not sure if Gene bench tested your model Marantz, but it should be more than capable of swinging those voltages from it's preouts. You will have to crank the volume higher.)
