Firstly, I am not aware of a stacked 250K/250K audio-taper pot. Stacked pots are either 250K/500K or 500K/500K.

Now, my brain isn't in gear this morning, but if I'm thinking at all straight, the load of two 250K pots is 125K, and the closest to achieving the same load with 3 pots is either, a) two 250K and one 500K, which provides a load of 111K, or b) one 250K and two 500K, which provides a load of 139K. Using three 250K pots would provide a load of 83K.

As to my thinking of using a 500K pot for the Q-Filter as part of the above equation, I'm thinking the slight increase in "gain" and "range" offered by the 500K control will offer a slightly broader range of adjustment. The volume and regular tone are left as 250K so that the tapers remain as expected.

Aide toi et le ciel taidera (Aid thyself and heaven will aid thee)