I think your "quick simulation" is just a bit too quick. First off it's not clear just how much of the circuit you are including but the pickups always see the resistance of the volume pots in any case. And furthermore it looks like your extreme roll off is due to the 250k series resistance plus the cable capacitance. We should note that when your volume is set in the middle HALF the value of the pot is in series with the output. So there always is a problem with series resistance in that regard.
The bottom line with your simulation is that it's clear that it's best if the series resistors are kept to as low a value as will solve the tone interactions. Since without them the tone circuits actually short together when volumes are on full, it seems to me you could get a lot of improvement with just maybe 50k or 100k or whatever. Rs are next to free. Just try some different values and report back what you found is as low as you can go.
Notice too that when two pickups are shorted together in parallel it cuts the inductance seen by the circuit in half. This moves the "knee" of the roll-off of the R-L circuit to higher frequencies. This creates a problem like my OLP MM has where it's very bright with both vols on full but gets very dull as you turn both vols down. Even though it has a single tone control, I"m thinking that these series R's will help it too. Gotta try that.
There is a LOT of subtle "art" in getting a passive circuit in a bass or guitar to be optimized for tone. And we haven't even started talking about where those resonant peaks come from as an interaction between inductance, resistance and capacitance.
