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

@kijanki But that’s the theory. The reality is that all things being equal, more power can bring an authority to the music like a firmer grip and more breathing space for the music dynamics to fill. It is also true, that much of the action, at medium volume, takes place in the first watt. It all depends on whether the OP wants to crank it.

@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?