How much power needed to drive 91db speakers?

"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."

You would be surprised!

I use JBL 2241H 18" woofers rated at 600 watts continuous.  These were redeveloped from the 2240 specifically to overcome the effects of power compression.

At 600 watts, they loose 4.3 dB.

At 300 watts, (i.e., -3 dB), they loose 2.6 dB.

At 60 watts (i.e., -10 dB), they loose 0.8 dB.

As I noted, these are 18" woofers with 4" vented voice coils specifically designed to mitigate power compression and I can imagine that a "lesser" woofer would undergo more compression that this.

https://jblpro.com/products/2241H

I have speakers that are 89spl efficient. My 30wpc primaluna with EL34 power tubes has no problem with sound level in a 20ftx 15ft room. I have drapes covering an 8x6 window, fabric furniture and carpet.  The room size and what you have in it is a determing factor. With the power your system has you should be able to blow off your roof. 😆

I have an Elekit 8900 I use with 93db speakers NX Otica with the OB subs. I started out with 300b tubes and had plenty of volume and recently decided to try 3.5 watt 2a3 tubes. Surprisingly, it sounds better to my ear even with what most would consider an under powered amp. I don’t listen loud anymore, but it easily plays loud enough for my normal listening levels. I listen sometimes in the mid 70DBA range with measured peaks in the mid 90db range and it sounds unrestrained and enjoyable to me.

I am using a high pass filter so the signal to the amp starts rolling off at 80hz, I have plenty of punchy and defined bass and even watch movies with this setup and it never sounds strained, keeping in mind I don’t listen real loud anymore. I’m not sure how much head room I save from this setup, but I think letting subs handle some of the bass helps when using low powered amps that otherwise could sound more strained or run out of juice before reaching satisfying listening levels.

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.