Maybe it's just me but why in the world would anybody need more low end thump than the Herbert delivers with EL34 or KT77? It's a guitar amp not a bass amp.
I did it. For the correct bias they do get pretty hot. peter doesn't reccomend them anymore but theres lots of people here that still do it. I put 2 kt88s, 2 6l6gc, and 2 el34s in mine and it sounded pretty good. I like 4 6l6's and 2 el34s the best...