The mic/harp are going to drive the front end a lot harder than a guitar signal so using the 12AY7 will be an attempt keep some headroom. The 2 Stroke does not have much anyway. The power tube choice is more of a guess, the 6L6, 6V6 and EL34 all breakup differently, so trying them with a harp will be a good experiment. Adding more filtering to the power section seems like it might be good. Also, changing the grid resistor might be a help too.