So I was speaking last night with a good friend of mine, one of the most well known musicians in my area, regarding this issue when we use our selector switch. He has the same issue with his mid-late 90's G&L ASAT semi-hollow (lovely guitar). It has humbucker and a toggle to switch to single coil, 5 way selector. I was kind of conditioned to play with my volume knob full-throttle - he said uses his volume knob to compensate for the volume drop (eg, when he's on bridge or neck, he'll be @ a 90% volume and go to 100% to compensate for the drop when using them in series/parallel.. etc..)
I don't know why I've always been @ 100% ... but that's how I was brought up I guess. I probably have adapted to the volume drop and use it as a tool when playing in the background, when I play lead I tend to go bridge or middle for that extra punch. But what he said seems like such a simple solution, truly Occam's Razor... which is why I never figured it out, I have a tendency to overthink... another new skill to learn and then refine.
Nothing good has ever come from me thinking.. Thinking is when I get into trouble.