bias is not the problem here,for sure you end up with a variation in tone in 2 ways,one for the feedback and one on the phase inverter due to impedance changing and the fact that any voltage divider there will decrease HF without compensation.
But it is pretty useful if you use it down to half on a standard audio pot.
By the way the typical scheme used on Metroamp and others is a 220k pot with 2M2 security resistor.
Another system i found to be clever is the Frondelli MV,which is wired like a 5E3 Fender volume.
Anyway i would add a 250k pot in series with the stock feedback resistor.