How much power needed to drive 91db speakers?

No one did any math....91 db sensitivity would give you 101 db at 10 watts or 111 db at 100 watts. That is for the peaks in the music...if the peak levels you experience when you listen are less than 111 db, which they probably are, 100 watts is a healthy choice. That means the amp won't struggle to keep up with the demand. Chances are, at most real world and sane levels, 30-50 watts of high current amplification would be perfectly adequate.

What you are saying is true but as a rough estimate my approach gets you in the ballpark of what you are looking for. Speakers that are rated for much higher power than 100 watts won’t suffer significant power compression or at least they shouldn’t. How much there is is going to be depends on the dynamics of the music...if its heavy metal head banging stuff then this is a real possibility due to the lack of dynamic range in the music. Classical guitar would be a whole different ball of wax. I would expect a speaker rated for 400 watts to be able to handle 100 watts with little to no compression. After all that is 6 db less than rated power.