How much power needed to drive 91db speakers?

My speakers have about the same sensitivity as yours and when I am playing at 75 dB I rarely use more than one watt.

Forty to 50 watts is ample power to drive those speakers unless you listen very loud or they are for some reason unable to provide the expected current with the 4 ohms nominal load (which probably can go down to 2 1/2 to 3 ohms at its lowest point).

I drive a pair of ZU Audio, Omen Def Supremes with a Pass Labs XA 25 class A amp. The speakers are listed as 98 db sensitive and I am using my PS Audio Direct Stream DAC as a volume control (probable not optimal), the room is 24x21x8 and I do have 2 Rythmic 15” down firing sealed subs, my listening position is 12’ from speakers. I can reach 100 db sound levels at my listening position. As someone mentioned above Pass labs may be an outlier when it comes to power rating their equipment. Conclusion I listen to mostly rock and the wife will crank up the Techno(feels like I’m in a German Discotech) while working out and I have never wanted for more power. 

As others have said not all watts are equal- I was dumbfounded by an ARC VT-80 that was rated at 70 watts. It was more dynamic, finessed and detailed than several amps rated at 400 watts or more. 
 

Ideally try and get a demo with your equipment - ideally at your house. 

@billstevenson 

"Ask yourself what is it you really seek to learn."

I'm curious. Did you watch a lot of "Kung Fu" when you were younger?

As others have mentioned, power requirements also depend on the size of your room & volumes you enjoy. In addition, the type of music is a consideration. Quieter acoustic music generally uses little watts but music w/ big dynamics and / or big bass can use 10 times the average power on peaks which is the primary reason for truly needing hundreds of watts/ channel.