It isn't only the power tubes. It's also the preamp design, plus speakers. As is noted - the Pro is really a Tweed preamp tacked to a fixed bias power section. DR is a BF design with lowish preamp gain, and a scooped midrange. That preamp is designed for moderate gain and smoothness, premised around the reverb and tremolo. The sweetness of the DR is the power tube breakup. The Pro - turn it up, like a Champ you're getting preamp and power amp gain with lots of mids, coupled with a dirty-gerty power section. Small output trans, squawky speaker. These two amps are very different in every important respect. Forget the EL84 vs 6V6 - it's pretty minor compared to preamp and output transformer/speaker. You could fit a mid control get a cutout ort lift to boost mids on the Normal channel - fit it with Brown Fender or Marshall preamp valies. That would dirty it up a great deal, similar to the hot boost switch on Mesas. Changing the values to Brown would get you somewhere Marshally.