@mahler123 That's what it sounds like to me as well. I used to hear this effect at times, many years ago on prior systems. When trying to figure out what I wanted out of a system, I'd go from analytical to romantic, really all over the place, I tried so many tweaks, diy mods, changing equipment, all kinds of issues, this pinching of mids OP speaking of, just one of them. Simple change of capacitors could cause this to happen, say Mundorfs with silver, or silver anywhere for that matter.
I've not heard this effect in many years, even with a relatively high content of silver in my system today. So yes, one has to get system fundamentally right, as OP states, in order to not experience this pinching.
The only time I hear this effect these days is burn in , such as 300B tubes I'm presently burning in, finally, in last ten hours or so, bloom coming on. This pinching no longer an outcome, simply temporary position.