Nuvista Tubes - General


May I have opinions please on the pluses and minuses of Nuvista tubes versus vacuum tubes? Supposedly and from the basic homework I've done, Nuvista tubes purportedly offer high reliability, low microphony, low noise, consistency, and small size compared to their vacuum counterparts. So what's the down side? 
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I see that. And I hope I wasn’t patronizing! It’s just that they are such different beasts. And what is a tube sound anyway? ....
All good. And your question is well taken.
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Interesting bit of info as well that MF went to the trivistor when the Nuvistors dried up.
Musical Fidelity time-line http://www.musicalfidelity.com/about-us/timeline confirms that nuvista 1997 and trivista 2002, but why then (seeking speculation) would their current amplifier offerings utilize nuvistor tubes instead of trivistor? 
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I have owned the MF Nu- Vista 3D CD player for about 16 years. It is really a wonderful piece of gear. The sound is very "natural," almost analog in nature and is not at all "syrupy."  Pretty good for a metal encased micro-tube.

As for the Nu-Vista vs Tri- Vista issue, my understanding is that there were a lot of problems with overheating in the Tri- Vista components. I have read a number of posts on various audio forums attesting to that. The Nu-Vistas were designed to stay cooler than the average tube, so that may explain MF's return to its use.