How much power needed to drive 91db speakers?

No experience with Coda. An under the radar company that produces high current amplifiers you might find interesting. https://coda.cc/amplifiers

This is not a suggestion, just my experience.

According to Audio Science Review (I have no knowledge or reason not to trust his methods) my Carver 180 watt KT120 tube monos measured the most distortion he’s ever measured yet they sound absolutely stunning driving my very revealing Avalon Acoustics Eidolon speakers. Better presentation than my legendary sanctified Marantz 8B. 
As tube amps go the Carvers run cool and are easy on the power tubes. 
Good luck with your search. 
 

Two things for me when thinking about this. 1. Headroom, going from the PS Audio amp that has tons of headroom to something much much smaller that may not have that extra in the gas tank. 
2. Damping factor, going to small tube class A amp you might be lacking there compared to the PS Audio amp probably has DF in spades. 
 

But I think overall you should be fine. Your ears will tell you if the SF speakers will get enough to sing. 

"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.