My amp came with 6550's. I had rebiased with el-34's and they sounded pretty good. However last night I poped in two mullard big bottle nos 6v6's (not 6v6gc) and holy moley I didn't stop playin for quite a while. They sound fantastic in there. Its like a totally new amp. Any of you guy's ever tried running 6v6's in an older non-master four input marshall??? Much to my suprise the cleans and the crunch tones are excellent. Let me know what you think.