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

tls49 is correct in his analysis, but I suspect in a small room at non-head banging volumes the combo will work fairly well.  Tell tale signs would be harshness on peak signals -- think loud cymbals or orchestral crescendoes.  It will be subtle, but easily heard if present.

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.