At 485+ plate volts, I would say no way. Even normal low volume playing would fry them or take the OT out. The MK IV can handle the 6V6's due to the lowered voltage of the Tweed setting which puts it right in the proper range for 6V6's. The natural plate current and bias would be another story.
What a nasty old rusty bucket that thing is. The rust on the transformers has got to be getting through the laminate and to the core. A time bomb waiting to go off. I would not buy any amp with anything more than a ight surface oxidation that I can clean and seal. This thing looks like the Martian landscape. It is a very sad example of a great amp. Mesa should have dipped their transformers a bit better. I have one that was totally dunked in resin and looks brand new. The other ones have to be kept in a humidity controlled environment and wiped down or sealed with shellac. I use a product call HHS 2000 which is a super long life lubricant to seal the surface and they now do not oxidize under any condition. That thing looks like a basement classic gone awry.
