One of the reasons that tube swapping is a "thing" is that there are clearly audible differences between tubes. It's one of the aspects of the hobby where the hobbyist has some sense of control over the sound.
Some 30 years ago, I lived in Berkeley California, walking distance from an electronics shop that sold tubes, had a tube tester and a big NOS stock of tubes. I'd swap out tubes for my Stax Amp/Energizer [hybrid: J-Fets driving Triodes], later for an Ampex MX-10, [all-tube, ancient and very funky], Marantz 8B [glorious mid-range at the expense of everything else], and a 'starved tube' microphone preamp [another hybrid, op-amps driving the tubes, if I'm not mistaken]. Compared to the exclusively solid-state gear, the upper octaves of tube hybrid gear sounded more "rounded off" and there was a bit of soft clipping on peaks. A lot less 'grain'. The systems I'm using now, while extended as far as I can hear in the upper register, have few upper register resonances or audible hot spots. The Stax earspeakers were peaky on top, producing good "presence" and spatial effects. So were the speakers I was using for monitoring, little NHT bookshelf speakers. The Drop 6XX 'phones and the a/d/s 400 speakers in use now are as smooth as silk, driven by solid state gear.
The biggest difference with tube-swapping was in the upper registers. I'd use a Rat Shack tone generator connected to the inputs of the tube gear, connect the output to a cassette deck and look for differences in level. Also did this with various interconnects. There was a 5 db difference at 10K swapping out line level interconnects on the MX-10. The interconnects I made without shielding had the most treble, some old, thick Monster Cable stuff that was overloaded with di-electric had the least. I asked Jack Vad what was going on, he said "Send me a schematic". After looking at it, he called back, said "There's no cathode follower." I know what a cathode follower is now, didn't know then. This issue didn't happen with any of the other gear.
This all points to some of the more "Audiophilus Nervosa" aspects of the hobby. ASR would say tube gear has obvious, bad, distortions. I'm sure SET gear has oodles of distortion. I'm sure the kind of loudspeakers favored by SET enthusiasts have wonky frequency response. The sum total must be full of mismatched impedances and easily measured distortions. But all these "broken" aspects of design can be fiddled with endlessly until the golden-eared audiophile gets the sound he deserves for a price he really can't afford.