Lots of good thoughts in this thread.
My addition is ..... good power !
Seriously, I think a key issue for tube amps pushing any amount of distortion is the quality of electricity that the amp is getting. At so many gigs, our band usually is running off one or (if we're lucky) two sockets (and that includes the PA and lights !). I find my tone under those circumstances extremely variable, and often rather cr@ppy and bright. I am convinced that it is down to inappropriate power supply, and my amp simply not being able to draw the current it needs in competition with all the other stuff plugged into the same outlet. There are many other stories of folks having these issues, and sorting it out with a proper (ie. expensive) power regulator.
I don't get these problems at our rehearsal venues, which all have good, and multiple, power outlets.
I'm getting so frustrated with these issues at the moment that I'm actually in the middle of changing over to a clean amp plus pedals rig for live use - I'm hoping that I'll get a much more consistent tone.