Modications to the Dynaco ST 70?

I recently purchased a used ST 70 which was supposed to be stock. It had not been used in a while I was told, but that I should bring it back "Slowly", and after that it should be servicable.

That being said, I was surprised to find 6L6GC output tubes instead of the EL34's. I haven't yet turned it on and all the tubes are Rayethon Japanses types. I will be testing the tubes very soon. The rectifier tube looks like it is clear plastic! a sharp seam running right over the crown of the tube!

I have a few questions:

Using 6L6GC's, could the previous owner run these tubes without modification to the original ST70 circuit? If yes, would it have ruined anything?

Does anyone know if there is a reasonably priced mods available without removing the tube rectifier circuit..I'd like to keep that aspect of the unit. The newer ASL stuff is pretty good, so I don't want to pay that kind of money to mod this thing. I have been thinking of the Triode electronics input board, or the Sound Values input board.

Thanks in advance.