Anyone Tried Unmatched Power 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

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?

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

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

I've run mismatched tubes lots of times. Every time one blows another goes in for however long it takes me to get around to buying another set. Always do full sets. Because almost always when a tube goes its been long enough to do the set anyway. Once in a very great while if one tube goes and they're pretty new then I will replace just that one. In nearly 30 years that has only happened with the Tubestore.com Preferred Series, which when 2 out of 4 blow spectacularly within a month they became my Never Again Series! 

To answer the question no, I've never heard any difference. In fairness these are all Tube Store perfect match, they test and write a number and since the first one tried worked I always order that same number. But I didn't always do that and don't recall ever hearing any difference even then.  

If you're really concerned and out to save money the way to find out for sure is simply buy one or two unmatched tubes. Probably you will find no audible difference. Worst case you wind up with a spare or two.

I do this with a lot by the way, and not only with tubes. Everyone has their opinion, and they'll argue it a million ways to the end of time. When nine times out of ten its not that hard to try it for yourself and then you know. 

Also, I would assume that, in a four power tube amp, only pairs of tubes needs to be matched.

I agree with this. I was using KT77’s when one of them red plated. I bought a matched pair as a replacement for the right channel, biased them and everything sounded fine.

With regard to small signal tubes, I always use matched pairs. You can use a pair of unmatched tubes as long as they are from the same manufacturer, same factory, and same generation of that tube type; ie, a NOS RCA 6SN7GTB with a used RCA of the same type which has comparable measurements. Most likely there won't be any difference in sonics with the amp.

Be advised that the used tube will probably start to die-off before the NOS.