i've had squeal(internal feedback) when i use 1M pot guitars like a jazzmaster with my rat. all 250ks are fine and when buffered it's fine...its worse when dist and filter are higher.
I assumed it was an impedance issue, so i wanted to raise impedance at first, but in fact lowering it was much better at sqeua reduction-and simulates my guitar having 250k pots in parallel resistance.
however, would i have been better changing the input +4.5v with a 1M resistor to a higher resistance? this seems like it could work as voltage rails usually squeal if shorted, and as it determines impedance too might preserve tone more?? or would it affect gain to the opamp somehow?
-a second mod i also made was changing the filter, so that there is the pot in the path to ground with a 0.047uf cap. Its improved by having more useable rolled of tone-fatter, less volume roll off, but the pot only works at the last section of it's travel-with lpf on full. i left the 1.5k in there, but think it may help if that was removed-as long as its only part of the filter-not another function, and of course has an effect: as i notice other resistors in series with it on the schematic.