Tube Amplifier Sound Characteristics

+1 @knotscott  Spot on. Sort of makes most of the threads debating about amp brands look suspiciously like a waste of time.

@Hilde45 I was just thinking of tubes as supplied from factory or stock units. For example on the extreme left (warm/tubey) I find Conrad Johnson. I have a Rogue preamp that is more in the middle. I have heard from people that in general that is their house sound.

I would caution anyone thinking about older VTl....yes they sound good, but if the day ever comes that you need service you will experience the worst customer service on the planet.   

@oddiofyl I have heard, that is why I am reluctant to buy any older VTL. I do have a good tech to fix amps but I dont want anything too old.

Like any topic, it all depends on how much you want to dumb it down on the spectrum from peer-reviewed white papers at one end and Twitter posts on the other.
With so many variables at play I don't think a simple warm-cool linear scale is enough to make an informed decision. It might let you cross a few brands off a big starting list. 
A more useful but still digestible format might be similar to how some reviewers rate ~10 attributes and score each on a 1-10 scale. Even rating a few amps you know would take some work.

Honestly, I think familiarity with the nature of various tube types is one of the most critical aspects of what OP is trying to solve for, maybe more than the brand name. Joe's tube lore is a good source for small signal tubes, but I don't of anything similar for common amp tubes. 6550s, EL34s, OTLs w/6AS7s, KT88, KT120, KT150 and little EL84s all have their own characteristics that are a big piece of the puzzle. Sure, circuit design, other parts quality, etc. also contribute. 

Choosing tube amps without knowing tube types is like reading the Torah(they don't print the vowels!). It can be done, but is a lot easier if you understand. Cheers,

Spencer