How much power needed to drive 91db speakers?

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. 

@curiousjim. Speaker power requirements see here. 

Mike

The answer is in your room size and how loudly you listen along with the impedance curve of the speakers.  In a 14 x 20 room with a 90dB SPL monitor I use 55w of class a tube power and they are driven well.  25w from a SET mono I have is adequate in the same space.  In my 20 x 40 main basement, neither is adequate at reference levels. 

My speakers also have a nominal 6ohm impedance.  Usually speakers measures at 4ohms are less sensitive than the measured spec due strictly to the way it is calculated.  See any review for Tekton in Stereophile.  It is in how the math is done.  

so the answer is maybe, possibly probably but need to know more.