Replace the stock JJ tubes with Svetlana 'winged C' or the JJ Blue Bottles. Instead of the 6550 tubes, try KT 88 tubes, again Svetlana are a good beginnng.
The 6sn7 tubes are a bit more work, but you can do much better than the Billington tubes, or so I hear. I am currently using some Sylvania NOS tubes.
This amp is really different and most tube dealers don't have experience with them---especially with an amp that is running this combination of tubes.
The most reputable dealers will make suggestions but will concede they have not precise experience with this amp.