Low voltage is to ensure easy class A bias w/o running the valves to their dissipation limits. Makes getting replacements easier, after building an amp to use Type II KT90's ( and then having the supply of good ones vanish ), I have changed the philosophy.Besides, the power diff between 80% and 95 is not a big difference. 807's are not out of the runing yet, they fall into good 6L6 cassification for me.
regards,
Douglas