Speaker wants 100 watt RMS. Is 40 watt tube OK?

Please correct me if I'm wrong as I am not an expert but I have always felt that that the higher current amps give much better control over the speaker. Damping also plays a critical role. I feel like watt ratings although important are usually a marketing term to ill informed consumer.

Actually no speaker needs more than 20:1 for a damping factor and many need less. If the damping factor is too high the speaker is overdamped and an unnatural phenomena known as 'tight bass' occurs. Many audiophiles like tight bass, but its a coloration.

I can tell you on my experience using 50 watts manley stingray with thiel cs 6.3 and even with larger cs 6s. Stingray perfectly drived cs 3.6 s for many years. Later i used 200 and 300 watts solid state amps with the same speakers. I cannot say that there was a luck of power from stingray

Actually no speaker needs more than 20:1 for a damping factor and many need less. If the damping factor is too high the speaker is overdamped

True, above 20 it practically doesn't matter.  What difference could possibly 8/20 = 0.4 ohm make, when you already have about 6 ohm in the circuit (approx. 6 ohm is resistive impedance of 8 ohm speaker). 
Difference between DF=200 and DF=20 is like 6.04 ohm vs 6.4 ohm (6% difference for damping).   Difference between DF=200 and DF=2000 is even smaller  0.6%

The bottom line is “ You won’t know until you try it “. However , I went down this Rabbit Hole with a 30wpc integrated and speakers that needed 40 watts , but were more at home with 200. In my scenario it sounded fairly nice , but couldn’t play loud or have any Slam . You need a bigger amp/amps . Something like Rogue Mono Blocs that have a SS power supply and KT -120’s. Go big or be disappointed . The Manleys are somewhat particular , so read up . They would be great with some DeVore’s or some other dynamic , easy to drive speaker . A pair of Bob Latinos 120 watt monos would be sweet with your speakers and very affordable. Cheers , Mike B.