How much power needed to drive 91db speakers?

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.

I  happen to have focal Sopra 2  speakers  91db efficient  at 8 ohms 

I have a 45wpc Pathos inpole  integrated amp 45wpv 8 ohms and 75 wpc into 4 ohms  my room is big 30x 16x8 but I only sit 11 feet from them  .itcan playinto 95db peaks it weighs 86 lbs and a dual mono design  .the question is how  often do you need to play a tear damaging level ?   In class A it does get warm over 100 degrees  sonically superior to class AB  and the Patented Inpol technology it uses the 4 vacuum tubes for again stage to power the speakers as well as to the resistor ladder preamp section , and has dual linear power suppliesforthepreampsectiin to  big inductor transformers ,Mosfets, then potted torroidal transformers to the output section  . For me it’s plenty loud and very tunefull and detailed Pathos makes a great hand built product from Italy.

I was at a dealership yesterday chatting with two other customers who own very expensive 12 watt amps.  We were listening to custom built horn systems and I assumed that is what we all ran.  I was surprised that one ran a pair of 89 db/w tower speakers.  It wasn’t surprising to me that these amps can handle such speakers, it was just that he had $70k or more of amps and linestage powering so comparatively modest speakers.

Had a pair 4367. Woofer sensitivity is 94db with the mid/tweeter resisted to match. A seventy-WPC VAC amp and a 150-watt Hegel amp could not properly control the woofers. I had a sense of it at the time, but it was obvious after replacing them with  a 300 watt ML amp.  The issue wasn't room size or volume level but of bass dynamics and articulation. I now have 85db speakers which are much less sensitive between the Hegel and ML amps. I use this is an example of how I have never related volume, room size and sensitivity to amplifier power requirements. Before that may come in to play, I already hear bass problems. That's my experience, though. curiousjim already has a good reference point. 

My Pathos integrated Class A inpol powered integrated has a lot of power for most any speaker 88 db on up in a good sized room

https://upscaleaudio.com/products/pathos-inpol2-mkii?pr_prod_strat=e5_desc&pr_rec_id=14980c6b1&pr_rec_pid=7026281873592&pr_ref_pid=2517096005692&pr_seq=uniform