I'll take a crack at your general questions here, but since I haven't heard either of the two specific amps you mention, I'll have to leave that for others (I landed on the Valhalla 2 for my tube amp after trying a few).
First, don't over-think it. Tubes are just a different means to the same ends. That is, they are just another way to provide enough power to a speaker to make it loud enough for satisfactory listening. If you understand the fundamentals of a SS amp, as you mention, then you know enough to dive into tubes. For example, your question about the planars is at least partly answered by understanding output impedance. True tube amps tend to have a high output impedance. Planars tend to be low impedance and low sensitivity. A 32 ohm output impedance matched with a 32 ohm headphone sounds bad. Why? I'm afraid you'll have to ask an electrical engineer. But I've tried it, and it's true.
I think what you might be after is more like, "how do I know if I will like a particular tube amp's sound?" It's actually a pretty hard thing to know, because it's quite subjective. Most SS amps aim at transparency, and these days typically get pretty close. Tube amps, however, add distortion. But a lot of people find that particular type of distortion pleasant.
The good news is in the answer to your question about buying your own tubes. Most tube amp owners do, and experiment with different ones. Unlike a typical SS amp, you can change the sound that you get out of your amp. It can be addicting. All you need to know to get started is the model(s) of tube that fits your amp, and how to search the internet. And presume that you'll probably spend as much or more on various tubes as you do on the amp.
My general recollection from my research is that the two models you recommend aren't all that different from one another, and are both relatively warm and "tubey." Barring input from others who can speak to their specifics I'd say pick one the same way you would a SS amp, then start playing around with some of the cheaper tubes out there to hear the differences (they can vary from really similar sounding to obviously different). Just beware that it's a rabbit hole, but a fun one.