Speakers that are good with tubes

@carlsbad 1+ . Efficiency and high impedance. Horn speakers rule here. Klipsch Cornwalls are a favorite.

Speakers' efficiency does not need to be as high as 95db. Tubes do not like big swings in impedance. I have run speakers with efficiencies as low as 88 db just fine with tubes (e.g. Fritz speakers). My Ascend towers are 92-93 db and they're great with tubes, because of the relative flatness of their impedance curves. If this fact is understood, the range of speakers opens up a lot. The hard part is finding measurements of the impedance curves. But they're out there, or you can inquire or look for a review in Stereophile.

Some good discussion, here: 

@carlsbad

I listen at 1/4 to 1/2 watt per channel 98.44% of the time and most people listen below 1 watt when using sensitive speakers.

Good point , the wonderful and noteworthy aspect of easily driven tube friendly speakers. I agree with @hilde45 that the speaker impedance (Ohm load) and curve characteristics are equally if not more important than the sensitivity measurement in isolation.

Amplifiers work less hard driving higher load impedance speakers that demand much less current draw. The flatter the impedance curve, the easier it will be to drive.
My speakers are rated at 94 db sensitivity and 14 ohm nominal impedance (Minimum 10 ohm). My listening position is 10 feet away. At listening levels typically in the 70s db C-weighted I’m using less than 1/10 of a watt.

Charles

Relatively speaking high powered tube amps don't sound very good. If one is going to bother to search for efficient speakers with benign impedance curves I'd suggest looking at medium to low powered amps. Just my opinion of course,