Yeah, dropping input impedance via lower-value resistors or pots (like 500k) can cut cable capacitance noise and brighten less harshly. 1M is standard for vintage warmth, but it's not sacred—modern rigs often run hotter with 250k/500k. Experiment.
Discussion
I think the issue may be due to the length of wire from guitar input to ground or elsewhere. The input is not directly attached to the chassis, and a ground wire is run to it instead.