Nuvista Tubes - General

I take this to mean your statement about a hybrid being a sales pitch ** is ** specific to MF.

No there are others that do it, just to get the Tube guys interested as well.
Just like McIntosh throw an output transformer on the end of what "could be" perfect good solid state amp, absolutely no need for it if the solid state amp is "done right". Try putting an output transformer on the rear of a Gryphon, Krell, ect, and see what happens to the sound. 
 
Cheers George

Heard the Nu-Vista amp back in the early 2000s when it was new.

Opinion?  Good but not something I'd lust after. It was demo'd on unfamiliar speakers and source, however.   Plenty of "air," if that's your thing.

Per OPs question: another (older) use for nuvistors was for the front end of VHF receivers/preamplifiers and were employed in my (early 60s) ham radio equipment. The smaller size meant lower inter-electrode capacitance and therefor higher efficiency. Reliable low noise transistors for higher frequencies were rare and expensive then. Other than that (as already posted) they are just vacuum tubes. Thanks.

@fraterperdurabo and @andy8400

Thanks for the candid feedback. My curiosity with the Nuvista (nuvistor) is that within the realm of what I already have (MF M6si) it is Musical Fidelity’s "upgrade" offering (with respect to where I’m at now). But I just can’t wrap my head around how much improvement there would be, at more than twice the MSRP mind you. Sure, auditioning is worth something too and if it comes to being a more serious consideration I would do that. Typically though I’ve found auditioning components (speakers and basically anything) is only so efficient, viable, meaningful, etc. so I honestly give more credence to opinions of owners, manufacturer marketing hype, technical specifications and overall product reputation.

Given what has been stated in this thread it would seem nuvistors are worthy of consideration, but certainly not the be-all-end-all. I was perhaps hopeful to learn or field opinions that it could offer traditional vacuum tube sound without the negatives, whatever the traditional sound (if any) and negatives (if any) happen to be. And its already been pointed out (here and/or elsewhere) that there is no tube sound per se and everything has negatives.

In all likelihood if and when I do "upgrade", I would probably be inclined to get something along the lines of a class A Pass labs amp just "because". Class A (seemingly) always gets the most up votes as far as sound quality and Pass always the same kind of thumbs up. But for now, as the Rolling Stones might say, "I’m just sitting on a fence".