When looking for suitable speakers for a tube amp - mine is a PrimaLuna EVO 100, 40w x 2 from EL34s - what must be looked at? How to best interpret the impedance pattern, whenever available? Assuming that one listens at normal levels in a medium-sized room, what are the criteria for matching speaker and amp?Look at speaker sensitivity. Even this is a gray area. My cutoff is 92dB. I simply will not consider any speaker with sensitivity below 92. However, a lot of that is because I enjoy a nice high volume level without having to buy amp with 100-200 or more watts. One time recently we had some really inefficient little speakers in here, 87dB I think they were. My 20wpc Raven Blackhawk not only had no problem driving them to a nice loud level, it made them sound a helluva lot better than a 100wpc ss amp that was also used. Enormously better. So again, gray area, rough guide.
I have for nearly 20 years now used all tube integrated amps, 20-60 watts, and just like your Prima Luna. Currently using Tekton Moab, and if you go with any Tekton speaker you will be fine. Better than fine. Before this I used Talon Khorus which were right at my cutoff 91-92dB and they were fine. Impedance? Who cares?
So brief as it was russ69 totally answered your question. Beware the people who recommend paying a lot of attention to technical specifications, because there is no end to it. Next you will have to learn to read waterfall charts, dispersion patters, obsessing about front vs rear firing ports, cabinets vs open baffle, dipole, planar, on and endlessly on.
Instead of what actually works: filter inefficient speakers out, then focus on finding the best sounding from whatever is left. Yes it really is that simple.