I have the same amp and I'm about to tube roll as well. one thing to note if you are using the balanced input the first small signal tube (closest to the input side) is not in the circuit. You still need a tube in there for the tube heater circuit so leave the factory one in there. That tube is for converting the single ended input to balanced. not used using the balanced input.
as for what tubes have the most effect I've not go that far yet myself maybe someone else has. personally i tend to keep them all the same regardless when i swop them out. I personally like 6550 over the KT88 but that's personal taste.