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

Tube watts vs SS watts; who you gonna believe; a formula or your ears?

@riccitone, what is delivered to speakers on average is in order of few percent on peak power.  It is because assuming half of the loudness (on average) means 10% of power, not to mention that music has gaps.  In addition power specification is very vague.  It is possible to find amplifier that can deliver huge peaks but is rated for lower power because of heat dissipation.  Such amplifier might sound much louder than, for instance, class A amp with smal headroom.  It also depends on the type of music you listen to.  Jazz trio music will need very little average power while heave orchestral pieces might need much more.

@noromance, true but it might be related to quality and type of the amplifier and not the power.  People might replace low power amp with better quality higher power amps or replace tube amp with SS.

@br3098  Damping Factor is pretty much useless for the "damping" alone.  Theoretically the highest real damping factor you can obtain is about DF=1.5    When speaker membrane moves on its own it produces back EMF (of the same polarity that would otherwise cause this motion) but current direction is the opposite - from the speaker to the amp, causing opposing force on the membrane that stops it.  This current circuit contains speaker, speaker cable and amplifier.  Speaker, that is in the circuit, has about 2/3 of its impedance resistive.  For 8 ohm speaker it will be around 6 ohm at low frequencies.  There are amps that sound great with DF=1, but if you don't want to make it worse (for the purpose of damping) DF>20 should be sufficient.  

@kijanki 

Great explanation, thank you! 
And in my case, definitely acoustic and jazz through very efficient speakers and a SS amp (considering some tube monoblocks!)

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.