Sounds like you got a winner. There is nothing wrong with the Sub Out. It does not adjust level, you do that on the sub not the amp. All the Sub Out does is pass the input signal to the sub. It is outside the pre-amp circuit. That is why it works even when the amp is off.
Warmup for any amp of this quality level is more a continuum than a destination. My Melody is not as good as a Raven, but I turn it on half an hour before listening, run some demagnetizing tracks through it, and still the sound gets better and better for quite some time. I would say the "warmup" is something like a matter of minutes to sound pretty good, an hour to sound really good, and then who knows how long to finally top out. This is of course a combination of not only the amp quality but the overall system transparency and listener skill level. Many people will not notice anything different after 20-30 minutes.
Your amp here however, while used it sat idle who knows how long. This first time being fired up it may be a little longer coming into full form. Not a lot, but that is just the way it is with anything high end that has been off a long time, jostled around, exposed to temperature extremes, etc.
Taps are purely for convenience and to give you the option to find one that might sound a little better. Which tap you run means nothing as far as the amp itself is concerned. There is a technical difference of course but all that matters is if one does sound better, use that one. Regardless of what the speakers are rated or the amp says. This is sort of like loading a phono cartridge, you try different ones and go with the one you like the best.
I am definitely seeing the appeal in a good tube amp though, whoa. The openess, bloom and freq response is amazing. I think its going to have enough power too which is really really surprising...
Surprising to you, my friend, and some others. As for me? Situation normal.