Matched tubes in preamps

Any two tubes of the same family will work(one in each channel), but- if their transconductance is not matched, your channel balance will be off. How much off depends on how far off the tubes' gains are from one another. Even slightly off will affect your imaging, soundstage, etc. Also, as mentioned: Mix brands and you probably won't enjoy the result. No more than most preamp tubes cost as matched pairs: Why even consider passing up the option?

I had never heard of someone intentionally mixing/mismatching tubes either, but that's how the literature for the Minimax reads. Perhaps it's curcuit design differs from other stereo preamps? Now, in a non-stereo guitar amp it's perfectly fine but that's a completely different application.

Depends on what you mean by 'matched'.

If you mean 'two tubes of the same manufacturer that are elctrically identical (tested/matched)' there are a few high end balanced preamps which will benefit, but will still run if the tubes aren't perfectly matched, but not optimally. Most pre-amps do not need to have 'matched' tubes.

If you mean can you mix brands of tubes of the same type, for example using a pair consisting of one EH12AX7A and one JJ12AX7A, the answer is yes, but its highly likely that you won't like the sound. Tubes brands and types have different sonic signatures as well as gain. It would be, sort of comparable to using two different brand mono amps, or even the same brand with different ratings. Why would anyone want to do that?

Hope that helps a bit.