Tips and advices on how to pick tubes for GM70 preamp

Hi,

Not sure if this is the best thread for such question, but I will take the risk...

I am a complete beginner in audio tube world. I have intermediate knowledge and experience in electronics, but I face tubes for the first time in my life. However, that is my goal - to learn how to build an audio system based only on tubes (not a product or any kind of homework, just sole knowledge expansion...) I want it to be universal, so the source could possibly be a 3.5mm jack, USB stick or even BT, Ill take care of the digital part myself, but how about:

I have heard that GM70 are really powerful tubes used in the last stage of any amplifier. However, can you suggest me, what parameters should I consider (or even better) or exact tube part numbers should I use for lets say first stage (or even first and second stages, considering the GM70 to sit in the third stage) preamplification. I intend to use 2x GM70 in a push-pull configuration for the full audio band. I guess I am looking for an answer that no book could give me, I am looking for pure knowledge and experience, considering audio tubes to be somewhat beyond regular electronics...

Some of you will say that this project is not for a beginner and I should start with something way easier. But Ill take the risk, as I mentioned, the goal is only the knowledge and experience, so time and money (to some extent) are not an object to me.