Tubes do not like big swings in impedance
What @hilde45 said, sensitivity yes it is important but the speakers with tubes (in general) won't shine if there is either an impedance difference or in general impedance under 8 ohm.
I do have a pair of Klipsch P37F, nominal impedance 4 ohms, only my Atmasphere MA-1's were capable (@120W output) to get them to sound decent. Other tube amps couldn't do it well. The day I put a solid state amp with those speakers there was no turning back, these speakers are made to match a SS.
Note that I love tubes, I'm not comparing tubes to SS
Try whenever possible to look for speakers that are at 8 ohms and over.
James Romeyn / Duke LeJeune / Hans Looman were involved with a recent design presented at CAF of a phenomenal speaker which if I'm not mistaken will be very friendly to tubes but you would have to look more into the details.