Often, "loading down" a pickup can help to tame some of the brightness. This is why you will often see the recommendatio to use a 250k volume pot for single-coil pickups. I don't know if your switching arrangement can accommodate it, but a simple fixed resistor between the input lug and ground on your volume pot might help alleviate the excessive brightness.
So, if your volume pot is 500k, try a 330k or even a 220k resistor, as described. 330k will simulate what having a 200k volume pot would be like, and 220k will simulate what having a 150k volume pot would be like. I might point out that sometimes more advanced preamps will include such switchable functions, so it is not that farfetched a suggestion.
Alternatively, one can solder a small-value capacitor between the input and ground lugs of the volume pot. This will selectively filter off some of the high end. Without hearing your pickups, it would be hard to make a specific recommendation, but consider starting off with 220pf.