Amplifier Power tubes - important to sound?

Well that is a perplexing concept… engineered in a way to prevent the tube character coming through. The only way I can think of doing that is to just power them but not put them in the audio circuitry… so, make them for show only. 

I've heard that before - various incarnations of "rolling tubes in this slot won't affect the sound", because "they're cathode followers with 100% feedback" etc. Still haven't met a slot that doesn't affect the sound. It's directly in the audio path.

You can have components that are "voiced" for the specific tubes it ships with. Sometimes components have a strong voicing on their own. In these cases, tube rolling may not readily yield satisfying results - but it WILL affect the sound. Always. Now is it a useful endeavor; a good use of time & money? That's for you to decide.

It's not surprising some manufacturers would discourage rolling. When you owe warranty support on these heavy, complex amps - do you really want users rolling them with power tubes willy-nilly, from unknown sources and questionable matching? Plus they may sell their own branded (labeled) replacement tubes, at healthy markup. 

"In the chaos and frenzy for something ever newer nobody stopped to ask if it was actually better."

It's better in a lot of ways but some of them are not related to the sound of the audio output.

Do any tube sellers match tubes at real world plate voltage and current under load?

I'm sure Jesse does. This is the greatest place to buy tubes in the entire whole wide world