I used to drive a pair of B&W 805s (sensitivity of 87) with my Cary V-12 switched to 50 wpc triode most of the time, and I always thought it did fine. It has been so long ago since I have listened at ear bleed levels that I cannot remember if I (always) went 100 wpc ultralinear or not. I know that some of thee times I did.
About a year ago I put a pair of Revel M126BE's (rated at 86) in my system and I was liking it in 50 wpc triode, but a while ago I decided to try 100 wpc ultralinear and I found that I actually preferred that effect. But it may have just been the 'something different effect.'
I would THINK that 50 wpc of good power should work well with speakers rated at 91, but the more I read the less I feel I know. I am now almost thinking that maybe the speakers lowest impedance may be more important than the sensitivity rating.