Is there actually a difference?

Last time I checked amps do not make any sound without speakers and vice versa.  

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.