Tube amps - what 3 things…

Nice sounding OPTs, and strong power supply are as important as anything.  The ability to switch between modes (triode, ultralinear, pentode) is nice.  The more tubes it can accept, the more options you have to tailor the sound to your tastes.  Enjoy!  

Now that we know the speakers, I would go with a nice SET amp.  You don't need lots of watts to get those speakers singing. 

Bias adjustment has been mentioned. Decide whether it is worthwhile for you to be engaged in that task. If you're ambivalent, a lower maintenance amp such as a PrimaLuna may be suitable. I've been happy with my Evo 400, and sleep soundly knowing the bias is taken care of by higher powers. But if you suspect that you'd enjoy the adventure of tinkering, go for it! I also had a positive experience with a Rogue Cronus Magnum, which has a built-in bias meter. Best wishes!

I don't get worked up about tubes wearing out, many of my tube amps from the 60's and 70's still have original tubes. I don't listen to my system more than a couple hours a day, so I haven't seen the tube consumption issue many reference. I don't know much about the brands you mentioned, but I would look toward older proven designs. There are some really great tube amps from the 50's, 60's and 70's, that with the right updates can be very reliable. Long term serviceability is the key to maintained value. Original McIntosh, Manley, Scott, Eico, Pilot, Fisher, Quicksilver, Manley, and a bunch of others hold their value really well. They are also serviceable long term, and are made with quality components. My 3 things would be:

Match the amp to the speaker - the Zu's will open the door to lower watt options that can sound very nice, but might limit your speaker options down the road.

Serviceability- look towards amps / manufacturers that have been in business a long time

Power tubes used- my preference for power tubes are E84 and 6L6. I like the sound of these tubes, and they offer me the power needed to run my not as efficient speakers.      

I too like EL84 and 6L6 tubes.  The KT66 is pretty similar in sound to the 6L6.  The EL84 must be an easy tube for builders to work with (probably allows for use of cheaper output transformers) because that is where you most often see it being used, but, I like the lively and punchy sound that tube delivers without being harsh.  The 6L6/KT66 are terrific sounding tubes--rich, yet plenty of clarity and good amps running those tubes sound "big" (enveloping soundstage and sense of weight and scale). 

Of the small triode tubes, I like 2a3 and 45 the most.  But, of course they do have limited output and cannot be used everywhere.  

I don't know about the tubes that are used in the output transformerless (OTL) amps I've heard and liked.  Unfortunately, there aren't that many makers of this type of amp because many tend to be unreliable.  That left Atmasphere, a very reliable brand, to this field.  The Berning amps (including the Linear Tube Audio) are sort of variants of the OTL (they are interesting, but for my taste, they are not voiced the way I like tube amps).  Anyone interested in tubes MUST, at some point hear an OTL amp--for some it is like a religious experience--the dynamics and liveliness can be shocking.

With tube amps, more so than solid state, you really have to hear what you are considering for purchase because of the wide range of sound.  While I am a tube electronics person, I find that there are plenty of tube amps that I would not want and I would get a solid state amp instead.  This is particularly the case with high-powered tube amps.  If I really need that much juice (I don't because I don't listen at high volume in a large room), I would consider solid state instead.