Stepped Attenuator to replace Potentiometer.

Hello: We have a set of Wolcott Monoblocks, for which I am researching replacing the volume pots with something like a DACT stepped attenuator for each mono block. The problem is that I can not seem to find a compatible attenuator that could replace the stock 25 meg ohm pots.

The circuitry is point-to-point wiring, powering eight 6ca7, two 6dw8, and two 12at7 tubes per side. It might be more of a hassle to mess with getting around the stock pots to substitute attenuators; however, I'm not an engineer and would therefore appreciate your informative advice and suggestions. Thank you.