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

@mijostyn +1.  It may sound OK at low levels, but you will have no headroom, smaller soundstage, weak bass among other issues.  Not having the headroom is the biggest issue for me.  Also, the idea that tube watts are somehow more than SS watts is pure nonsense.  

Forgive my engineering ignorance....but isn’t there also the consideration of how much wattage is actually utilized on average? I’ve generally thought enormous power “on demand” may not necessarily guarantee a firmer grip on the signal - or be needed - if the circuitry of the amp can allow it to peak and sustain well, and your speakers are rated for a decent relative efficiency. Btw, I’m not asserting this idea, more asking here! 

If the loudspeakers are 87db sensitive then the Manley amp at 40 wpc should be able to generate roughly 101dB at the listener position.  If the average listening level is 80dB, which is loud, then there is 21dB of headroom.  That's a lot of headroom.  Even if the amp is run in triode mode outputting 14 wpc the listener still has 16dB of headroom.

The Monitor Audio loudspeaker is a 4 ohm load, but it's not really hard to drive.

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.