Most likely power tubes. How old are they and how often do you play?
Power tubes do tend to go ‘dull’ with use over time, and it’s a gradual change so it goes unnoticed until it’s obvious. When I giggled and practiced regularly (about 40 or so hours a month) I changed them yearly, so at about 500 hours. I have the opportunity to play my amp at stage volume regularly so the tubes do get pushed. I alway find that my Roadster sounds clearer and more defined with a fresh set of power tubes. I chose that interval as insurance, as the power tubes get older and beyond their service life you run the risk of a tube failure that could take out components in the amp. I’ve seen power tubes fail and take the screen resistor(s) out with them.
I’ve also seen amps with decades old power tubes still ‘working’, but as soon as you put in a fresh set you’ll see how bad they really were. I had a friend with an old Fender he inherited from his grandfather. I think they were the originals. As soon as he changed power tubes the amp really came to life.
I recommend you change your power tubes every 500-750 hours depending on usage.
In contrast preamp tubes tend to work or not, or suddenly create pops, crackles and strange sounds when they fail. It’s not a gradual thing like power tubes.
Your amp is designed for 6L6GC or EL34 power tubes and their variants. EL84 will not fit.
I’ve tried 6L6GC, 5881, EL34, 6CA7, & KT77. I prefer the 6L6GC, and the JJ is a good choice.
I also tried 6550’s, but my Roadster is modded to handle the increased currents of the 6550. I wouldn’t recommend that with the factory power transformer. They sound much better in my modded Marshall DSL anyway.
Where ever you get your tubes from be sure to let them know what amp they are for, any tube vendor worth your business will supply tubes rated to work properly in your amp.
Dom