Funny you should ask about the GEC 6080, hopefully that will be my next port of call.
I'm pretty sure my GEC 6AS7G has it's best days behind. Even so there is a wider soundstage and slighly more air with the GEC 6AS7G. The Mullard is definitely a great tube for the money though. I find with the Teton it is the driver tube (6SN7) that makes the biggest changes to the sound.
Will have to see if I can source a new GEC 6AS7G without having my pants pulled down.