Speakers and vacuum tube gear

I would say that power, in a sense, is a factor of compatibility. To answer your first question, higher sensitivity speakers would be a good start. Beyond that, I think your question is too general.

Ho big is your budget? How much for the amp? etc.
Yes of course there is fidelity between tube amps and speakers. I agree with Mr. Reynolds the best speaker regardless of the tube amp in question is one that holds a steady high impedance. However the trump card is forever and always the speaker you like best. If you want an example how about dual 7-8 inch woofers each ported with 5.5 inch midrange and the inverted tweeters separately compartmented on my JM lab Electra 936s. Their rated sensitivity of 91 db/w/m and a promised impedance of only 3.6 ohm but nominal 8 ohms. Doesn't look like a great match but it's sounding better than any system should using Opera Consonance 800 monos for amps. They are rated as all of 78 wpc but have a 4 Ohm speaker tap on the output transformer which is what I use. Nevermind the four large bottle 6CA7s and the 80 lbs of mostly tranies per side.
My even less potent DA-60 again with 8 output tubes and rated at 60wpc doing a great job on VR2s a floorstander rated sensitivity of 88db/w/m which holds a very steady 8 ohms and over.It sound very nice and dpoesn't have any problem getting loud.
Some speakers are built intentionally for use with SET flea watt tube amps . These are frequently Single Driver back loaded horn designs to max the bass. Horns of nearly every stripe can be used but without active woofer amping are tonally imbalanced.
The question though is do you really want that sound?
Thus again learn your real preferred sound and build a system to suit.