1/ I can't see why your amp tech friend went to 150 ohms on that cathode R. Perhaps trying to panel-beat a dead 6V6 into delivering some power? Not the thing to do. I'd go back to the 470R, dont' worry 20 or 22 or 25 uF but I would select a 50V rated cap as the voltage may exceed 25V at times when you're really honking a big signal thru the amp. Also put a little distance between the R and C so the C doesn't get baked by heat from the R. Even if it's as little as 3/8 inch.
Or his vision is poor & he thought is was a 450 ohm resistor. Seen that sort of thing happen. Imagine a tech who's color blind, and too lazy to reach for his ohm meter. Red looks like brown, and brown looks like brown. Resistors off by a factor of 10, no wonder amps don't work so well.
2/ I'd leave that circuit stock. It's very Champ-like and that's a good harp tone for many purposes. Youl'll need to mic it up & send thru the PA for a gig any bigger than a very small club. You may want to collect a few more amps for different tone and volume choices. I've seen people use everything from a Champ (South Side Johnny) to a pair of Twin Reverbs (Paul Butterfield) for harp. Many of the small Gibson, Sears Silvertone, Wards Airline, Magnatone, & similar amps are good harp candidates.