I have been putting together a tube system as my primary system for months and going through what you are starting. I’m now on my 4th amp and very dialed in.
Yes, you need 90+ dB, preferably 95+ with an SET.
For the SET you will need to decide if you are going to have subwoofers of not. Bass is the hardest thing to fill out with low wattage. If that isn’t terribly important to you (jazz of classical) then the task gets much easier. Many of the speakers being suggested to you are going to be weak on bass with a 6 watt system. I prefer a full range speaker and no subs.
I know it is hard to get past all of the bashing of the tekton speakers but you won’t find a better midrange than the array mids and you’ve said midrange is most important to you. I am using Encores in a relatively small room and they work great with low watts. If you prefer small, look at the Perfect SET.
One thing to keep in mind in a small room is heat generation. Some tube amps will make a small room very hot. Look at power consumption which is a 1:1 indicator of heat generation.
Jerry