Anyone Tried Unmatched Power Tubes?

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

Always follow what manufacturer claims. If you want them to last longer get them matched.
Depends from the design if they have to be matched in pairs, quartet or, whatever. When i had an amp with 8 output tubes i always got them quartet matched. Signal tubes should be matched in pairs also. 
G

Thank you for your input.  

I have an amp that has four KT88 tubes.  Am I correct in thinking that the two on the left are one channel and the two on the right are the other channel?

The four tubes were matched when installed but now two of them are acting up and I want to replace them. Do I replace one in each channel?  Either both inside tubes or both outside tubes?

You are correct each pair for a channel.
I would replace all four in matched pairs.
Keep the good ones as spares. Why two are arcing?
I do not know if it is a push pull design, if it is, no matter inside or outside as long as they are matched per channel. A good vendor can match tubes with min extra charge.
G