No, you don't want to mess around with the resistance of the pickups themselves. If you find the single coils to be too bright with a 500K volume pot, you could solder a 1.5 Mohm resistor across the terminals of the 500K pot to reduce its effective max. resistance to 375K. This would split the difference between the 500K and 250K pots and might give you a good compromise. It will alter the pot taper slightly, but I doubt it would even be noticeable.