160562 .... you are wandering in the same technical mind (pun intended) field that I found myself. What Kijanki said is spot on. You may find it helpful to retrace the steps I have taken to avoid the land "minds" I stepped on by pulling some of my posts.
One interesting post in particular dealt with "back impedance matching." IOW, output trannies work both ways. The output tranny steps down output tube voltage and impedance characteristics to the match speaker load. At the same time, a speaker's impedance characteristics, which vary as a function of frequency, are stepped up on the primary side of the tranny. This is the load the output tube "see."
As Ralph (Atmasphere) and Al (Almarg) have explained, output tubes operate best (i.e., low distortion and power delivery) if presented with a load (impedance) that falls within the tubes "sweet spot." Since speaker loads vary as a function of frequency, the output tubes may operate outside their sweet spot. If so, power delivery and distortion may suffer to some degree.
The optimum solution: if using a tube amp, (i) select speakers with a ruler flat impedance curve that corresponds to the output taps (e.g., 4, 8 or 16 ohms; preferably 4 or 8), (ii) )zero phase angle characteristics and (iii) high sensitivity. Good luck with finding a speaker possessing all of these characteristics at the same time ... and that sounds good.
Bottom line: life is about compromise and trade-offs. Ce la vie.