There are some amps that won't tolerate mismatched valves. Mac is one BUT in pairs as you said per rail helps. The ability to bias individual valves help.
An issue for unbalanced valves top to bottom, IS.. One large dual or one small signal valve sharing a L/R duty, and no way to change the gain for the left or right. (Just one example). Some have an auto bias that will make a 20% works perfect with a new 100% +... With no problem it seems..
I had/have VTLs they didn't care too much as long as the valve held the bias.
Regards
An issue for unbalanced valves top to bottom, IS.. One large dual or one small signal valve sharing a L/R duty, and no way to change the gain for the left or right. (Just one example). Some have an auto bias that will make a 20% works perfect with a new 100% +... With no problem it seems..
I had/have VTLs they didn't care too much as long as the valve held the bias.
Regards