I wasn't really clear when I said "modify".
Is there a tube socket adapter available to allow an EL84 to run in the 6V6 socket?
It's not really a collectible amp in it's present condition. I paid $100 for it in 1995. It needed a new switch. Since Fender no longer makes the originals, I had to sub a generic slide switch from Radio Shack, which required changing the panel mount screws. The original speaker had a large hole in the cone from the previous owner poking the electric plug in the back of the amp. It was replaced by a new 8" Celestion. It is the same speaker they use in the Marshall Lead 12 practice amps.
I'm not worried about it's value. I'm just looking to change the sound, but leaving leaving an option to change it back if I want.