I like these kind of threads, so I'll kick in.
I too believe that there may indeed be differences in source components and pres and amps, but that there are no
unexplainable differences. They can be accounted for somehow, and the explanation had better be reductive, believable, and convincing.
That being said, I battle medical woo in meat-space, and when I get home and kick back, I'm a bit more forgiving of what most here would consider woo in audio-land. See avatar for an example of what I use in one of my systems.
Despite that I'm willing to play with high output impedance tube amps, I don't believe in magic wire, caps, maple plinths, pyramids or super sources. My boring digital front end consists of a collection of players that span the decades, from an old Sony ES to a new BD player. Level matched, and in spite of completely different DACs and output stages, they are indistinguishable. When those players break, I'll move on and get a music server of some sort. I expect what comes out to also be indistinguishable, if not better, than what I get now. And it will be hooked up with cheap interconnects and speaker cable. My old cheap homebrew 12g cables must be thoroughly "burned in" by now, they're extra chocolaty.
I would love to see the different measurements between a tube amp, class d and a solid state amp...
This is old news, but it has been reinforced to me with some inspiration from Bob Carver and folks here at Audioholics. A while back, someone posted a thread about one of Bob's recent offerings on eBay. On the listing, Bob was answering questions, and the discussion was quite interesting.
Regarding tube vs ss sound, Bob gave the expected answer, that the big difference was the output impedance and resulting misbehavior, but he went a bit further than what I had heard before. He postulated that in addition to the back emf from undamped driver excursion, the low damping actually allows the speakers to pick up room response like microphones, which, since it isn't damped out, instantly becomes part of the signal. The effect would be more noticable as output impedance went up, so typical PP tubes would have a little of the effect, and high output impedance SETs would have even more.
So I had to check it out, since I have SETs in a bedroom system to experiment with. Their output impedance is about 3.2 ohms.
Such behavior is actually happening in my room. I hooked a multimeter to one channel, completely disconnected from the source but with amps on and gains maxed. While cranking the other side, sure enough, I was getting a signal in the unconnected channel, way down in the low milliwatt range, but there. Volume dependant, of course. It goes a long way to explain the 'bloom' effect of using high output impedance tube amps, aside from the low order but considerable amount of distortion they produce. It's a more natural sounding reverb than I could get with a processor, and does quite pleasant things to the soundstage. It also explains why the effect is exclusive amp/speaker connection, and can't be exactly duplicated with processors.
I don't see any reason why tube circuits handling low level signals would perform any better or worse than their ss equivalents, unless by specific intent. I've tried a few, and just don't see the point, unless you're addicted to tubes for their glow. I guess I like my signal clean up to the point that the SETs/speakers bastardize it, and not before.
This particular corner of audio-geekery is a dead-end, I'll admit. But that doesn't mean that my weird franken-system that breaks all the rules doesn't get it's share of "Wow, that's amazing!" type responses. And that's all that matters, if it connects the listener to the music in an emotional, engaging way. Otherwise it's just noise.
Regarding Warp and his buddies' experience, I'm kind of with Peng and the others, in that with more careful control some or all of the perceived differences may indeed be illusory, but they are explainable somehow. I wouldn't be so quick to discount what they're hearing; the ear is the final judge, after all, even if it is influenced by subconscious biases. There is something aesthetically appealing about a short, simple signal path, even if a more complex one is sonically indistinguishable, and that alone will affect the listeners perception.