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

That idea that 3 dB is a very slight change in volume is ludicrous. 1 dB is a slight change. Raise one channel 3 dB and the other channel will disappear. The image will shift all the way to the louder side. 
Damping factor only matters at very low frequencies below 100 Hz.
Back EMF is highly overrated. Yes the speaker is a generator but a weak one. Only very large subwoofers with huge magnets might present a problem but the SS amp is protected by it's output stage's very low impedance and the tube amp by its transformer.
Power is power. Tube amps my clip more gracefully but any clipping is distortion. 
At any rate subwoofers require a high power amp with a high damping factor (low output stage impedance) Above you can pick your poison and power is not as critical. 

Agree with @mijostyn but wish to add one point. Blown speakers are almost always the result of insufficient power. When an amp is driven into distortion it causes stress on the drivers leading them to overheat and in some cases fail. It's true that tube amps clip more gracefully but I would err on the side of more power. Monitor Audio probably has a good reason to specify 100 watts of power.

I have seen speakers being driven with too much power, as well, causing destruction to the drivers, and sometimes, crossovers. Lets keep in mind, that the volume control setting on a preamp, does not determine the wattage used, of a connected amp. Many factors in determining proper power. Speakers being used ( size, relative to room, sensitivity, and impedance ); room size ( distance between listener and the speakers ); specific music being listened to ( a single cello and violin duo, or a full, grand orchestra, consisting of canon shots, just as examples ); maximum volume desired by listener; room acoustics ( too much furniture, over damped room ); voltage of source, feeding the amplifier; how isolated is the room, from extraneous sounds and noises. Tubes vs solid state, as speaker amplification.....I will stay out of it, because I favor good quality ss....

onhwy61  said: "If the loudspeakers are 87db sensitive -  Monitor Audio loudspeaker is a 4 ohm load." 

That is not correct the speakers are 90db and a 8 ohm load, 40 tube watts per channel should be plenty if not in a large room or at head banger volume.

Lot's of good info in the responses.  As a tube amp designer my 2 cents worth is that no mention of power supplies yet--and that is also a very key factor.  An excellent choke driven power supply can deliver instantaneous power which can be much more important than total RMS power.  Every amp is different and a 10W tube amp might outperform a 100SS amp, or the reverse depending on the total design.  

I once built a custom guitar amp for lead jazz guitarist which could only deliver 4-5 watts of continuous RMS power.  However, due to the design, it outperformed a 25 watt Fender amp and became his favorite gig amp.

try it and let your ears be the judge