Oooh boy, that's quite the question to ask in this forum ^_^;
The rational answer, certainly prevailing here, would be that the amp should be neutral, so the music is as it should be, without coloration. And that a good tube amp should, and can be neutral like that. So what would be the point of swapping the tubes then. Or it is not neutral, in which case the changing of the tubes may change that coloration, but how would you know then which one is closest to the original intent?
For the record, I think due to the very nature of the tube amp, in which the tubes are active electric elements, it will alter the sound in some way (unlike cables etc.) And as such, I think the tubes themselves have a bigger impact on the potential neutrality of an amp than its construction (though that is a whole other debate, and I may contradict myself shortly..)
So, IMHO, as the acronym goes, based on a bunch of tubes I own and admittedly enjoy, on at least 6-7 different amps over the last few years, and one tube DAC (optional output):
It does depend on the amp to some extent. I have a nice Peachtree Nova with optional tube stage, and I have swapped the tubes 2-3 times, could never hear a difference. I have a Fosgate Signature, a non-OTL design and I could barely hear the slightest difference if I reaaally paid attention in a way that no longer let me enjoy the music, same for the Woo WA7 (I have been mystified by some the claims of audible differences between the few tubes compatible with it, some of which I have used myself...)
On the other hand, on my "big" Woos (the OTL WA2 and the (balanced) transformer-coupled WA22) some tube combinations are definitely noticeable, and some work better with certain headphones and/or types of music, Yes, it is coloration, but I listen to music for enjoyment and it makes me enjoy it more, or helps smooth over some of the not-so-good recordings. And it is usually a combination of tubes (rectifier, pre & power) that I have named something like "Straight Sylvanias" vs. "Bottled Rock Power" etc. But that's just me -_-;;
If on the other hand you asked me for example to recognize the difference between two properly working power tubes, I'd fail miserably... Since this is the Sound Science ghetto -_-, I can probably admit this: I think that most of the tube comparison lists here on Head-Fi are more of an exercise in creative writing (but make for an interesting shopping list). I very strongly doubt the authors would be able to clearly differentiate between the tubes of the same type unless they are colored to the point of near-defectiveness, and never mind matching up the lush prose describing its character with the actual tube, in any kind of blind test.
You will also notice an interesting trend that the more rare and expensive a tube is, the more magical it apparently sounds (kinda like burn-in, it magically only ever improves things, never changing for the worse). Can't remember the last time I read something like "I bought this 1940's Western Electric, put it in, and it sounds kinda meh..." And new tubes (even if they're not cheap knockoffs) somehow never have the same quality or "warmth" of NOS even if they're well-built & tested before sale...
So, my answer would be: it depends. Especially whether your amp is a type that is responsive to tube changes (kinda, the more tubes are involved, the more likely it is
You don't have to buy super-expensive tubes just because they supposedly make a bigger difference. And you should probably have a set of spares anyway in case the ones you are using go bad, so you might as well try something different...
And the only way you're *guaranteed* to audibly recognize a certain tube in your amp it's the one going bad
