"Can't you approximate the effect of a 250k pot by just turning down the volume ever so slightly?"
NO. For two reasons. One, a volume pot is NOT a rheostat. The load the pickup sees to ground does not change. It is wired to the hot lug, and the hot lug is not soldered to the wiper as it would be in a rheostat. Second, when you turn a pot down, it introduces a series resistance into the equation, i.e. the resistance between the hot lug and the wiper. So when you turn a 500k pot down to about 7.5, which if it's an audio pot would be about 250k from wiper to ground, the pickup still sees 500k, and there is a 250k resistor now in series with your pickup's output on the way to the amp. Not at all the same as a 250k pot turned up all the way.
I use various resistors across pots which I have measured to get a resistance I want. That goes from about 200k all the way up to 1M or more. Various pickups in various guitars sound better with different resistances. PAFs to my ear generally sound like icepicks with anything approaching 500k. 225-300k works much better. JB? Most guitars like them between 250 and 400k. Hotter, darker pickups generally need closer to an actual 500k MEASURED resistance (not just the number on the casing which means F all). On neck humbuckers, I use 1M religiously unless they are unusual types like 40ga wire, firebirds, etc.
So, since I've actually done this more times than I can count, including wiring guitars so each pickup selector switch position sees a different resistance, I'm NOT asking whether anyone thinks it's "ok" to do that.
