FWIW, I have Quicksilver 60wpc monoblock tube amps and at first I thought I had to have high sensitivity speakers. I did try some at 92, 93 db and they had an easier time than speakers I tried at 87 and 88 db but I have to say I found that the Monos were perfectly fine powering speakers as low as 87db. They sounded delightful and not at all underpowered. I do suspect that a lower powered tube amp might have had more trouble, but my QS has a honking transformer and easily controls those lower db speakers — which I liked better. Impedance, as lowrider57 said is very important. I had 92 db Focals which had a bad dip in the 4 and sub-4 ohm region, and that proved a problem with the sound, in my setup.
As others here have said, this is a huge topic, and I will admit, I am a newbie. So all I am saying is that I *thought* I had to rule speakers "under 90 db" out of consideration -- and I found that proved false by actual experimentation. Also worth noting: in my space, I only need to play speakers about 8 feet away, at about 80db volume to sound good an loud.
As others here have said, this is a huge topic, and I will admit, I am a newbie. So all I am saying is that I *thought* I had to rule speakers "under 90 db" out of consideration -- and I found that proved false by actual experimentation. Also worth noting: in my space, I only need to play speakers about 8 feet away, at about 80db volume to sound good an loud.