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

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.