"Would you really need to stack LOP tubes? "

I think the max plate voltage of a 6MJ6 is about 5 KV, so it wouldn't quite do it, even bridged, I think.

"The most common way to do reduced power in the early 70s was with an HV cap & HV switch."

You mean like a lower value cap for the lower power level ?

"It's how they used to run bright emitters in the 20s. "

What is a "bright emitter" ?