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

According to Benchmark Media FAQ distance from the speakers matters only at the open space. Inside of medium size room we can assume that 1W (2.83V@8ohm) will produce 90dB loudness with 90dB/W speakers. Each 10-fold increase in power will result in 10dB loudness increase (twice as loud). 100W amp with 90dB/W efficient speakers should produce 110dB. Do you need 110dB? Do your neighbors need it?
100W is 30% louder than 40W.

(Perceived loudness = k^(1/3.5), where k is ratio of power)

@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.  

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%