Is there actually a difference?

There are so many variables that can screw up such a test. The speakers being a big issue. Some speakers are dark, dark speakers will hid variations. If you find a good full range driver, you have no crossover issues, and assuming that the speakers are articulate, these should make it easier to identify differences in equipment assuming that the high end gear is truly articulate, expensive equipment doesn't guarantee that the equipment is articulate or neutral, it only guarantees that it'll cost a lot. In my system non audiophiles can consistantly identify different gear, be it amplifiers, most cables, etc. Mind you, they don't always prefer the better (more neutral) items. Just as some audiophiles prefer dark sound some neutral, and some prefer bright equipment. I'm sure that some people couldn't tell any differences, but I haven't found anyone who didn't hear changed, when there actually was a chance. Then again, I use Teflon V-Caps, or Russian caps, Mundorf Silver Gold Supremes, and even particular resistors, so my system is tweaked to the max. Only my Yggdrasil is stock.

Why doesn’t someone else use this approach??

No-one makes SITs. They can be a bit fragile too!

I'm not sure input capacitance is an insurmountable issue.

I'm not one of those who automatically cringes at feedback.

I'm not either, but one thing should be abundantly clear at this point- even though tubes have their weaknesses, they are a lot easier to use to build a circuit that makes less of the distortions that the ear finds particularly offensive. Funny that no-one has stepped up to the plate to fix what's wrong with transistors!

If the industry had a weighting system on the harmonics produced by an amplifier, we'd probably see some change. We've known since the 1930s how much more sensitive the ear is to higher ordered harmonics than it is to lower orders, but little has been done about that in terms of circuit design- we've actually made it worse! That is why we are having this conversation right now: in a nutshell, the industry doesn't **want** to do anything about it, because it *costs too much money*.  So it puts its head in the sand, hoping somehow our ears will change despite millions of years of development :)  For example, if there was a range of SIT devices; signal, driver and various output devices, tubes would be gone. All done- history. But that isn't going to happen, so in all likelihood, if you have grandchildren that care about audio, they will be having this conversation decades from now.

Just for those unaware, SITs and VFETs are just power JFETs. 

There's always laterals, but nobody is doing much with those. Kinda like nobody did much with the big JFETs. The solutions to the problems transistors have has been solved and pretty much nobody really cared. Is this higher order distortion really a big issue? I'm not sure it is when it's kept at or below a low noise floor. The marketplace doesn't seem to think it's an issue. 

Regardless of the technology/topology, if YOU can’t hear a difference, there is none(in your universe).

In my opinion, it would be VERY easy to set up a double blind test where people would be unable to hear differences in various equipment. I would love to find a skeptic to work with where I pick the speakers, and the high end equipment. The speakers I would use would be 8 ohm, and over 90DB at 1 meter, not a difficult load, and very efficient. The difficulty comes into play with the DOUBLE blind part. Switching between 2 systems, level matching them, keeping the testers in the dark, and being able to immediately switch back and forth between gear so as to assist people in hearing differences would be the most difficult part. Anything truly random could have you listening to the same system after hitting the random selector switch 20 times before hearing the other system. I think a double blind start, and then allowing the listeners to swap systems at will would make far more sense. Honestly though I have strong suspicions that the naysayers have been allowed sufficient free reign to allow them to game the system. I think that the most important part of the test would be ensuring that the speakers are extremely articulate. I believe that there are many speakers out there that would easily mask differences. Other announces such as acoustically lively rooms, as in tiled floors, or excessively dead rooms, as in rooms with heavy carpet, and foamed walls could have an effect. I am not a room treatment guy, I figure music is played in imperfect rooms, but I do believe that in a double blind test too hot or too dead a room could color the results. I'd really like to be involved in one of these tests before ascribing any merit to them. I think that they could too easily be rigged otherwise.