Tube change on Fender "Evil" Twin

Hi,

Before I sell it and buy a Bassman or something, I want to see if I can make my fender '94 (evil) twin amp harmonica friendly(ish).

I read that Charlie Musselwhite played a red knob twin with no tube changes so this gives me hope that I can play with my twin till I decide on which to change to in the future, skills and experience quite obviously make a massive difference.

I've had the amp for 12 years and hardly used it, even with guitar. I recently had it serviced and have taken out two of the four 6L6 power tubes to bring the power down from 100w to, I think, 50w on high power which brings it down to about 13 to 15w on the low power setting.

It has five 12AX7 & three 12AT7 preamp tubes. I have just bought a bunch of 12AU7's and wondered which tubes to swap out to reduce the gain. I may need to get some 12AY7's also?