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

Lot's of good info in the responses.  As a tube amp designer my 2 cents worth is that no mention of power supplies yet--and that is also a very key factor.  An excellent choke driven power supply can deliver instantaneous power which can be much more important than total RMS power.  Every amp is different and a 10W tube amp might outperform a 100SS amp, or the reverse depending on the total design.  

I once built a custom guitar amp for lead jazz guitarist which could only deliver 4-5 watts of continuous RMS power.  However, due to the design, it outperformed a 25 watt Fender amp and became his favorite gig amp.

try it and let your ears be the judge  

What a great thread this is! Lots of well-written, clear information.

My suggestion to the OP is to read Roger Sanders excellent white paper titled something like "tubes vs transistors". IMHO the information he shares will also help illuminate the topic under discussion.

Is there some law of physics I missed in school? This may be sorta true in some cases, with some amps but it's far from a universal conversion factor. I can think of many circumstances with my own gear collection where this is absolutely not true.

The missing bit is distortion. The reason the tube amp can seem to play louder than its watts suggest is that they have much smoother clipping character- rounding the output waveform when clipping rather than chopping it off as a solid state amp will do. So you may not hear the clipping at first but with a solid state amp you will.

As the tube amp approaches clipping it will generate more higher ordered harmonics. Since the ear uses these harmonics to sense sound pressure, it will sound louder at that point but a sound pressure meter will reveal what is really going on.

I asked Monitor Audio a similar question when I owned Silver 8s, but I was considering a 20watt/Ch amp. They said it would be fine. One reason MA recommends a lot of power is because their floortstanders are capable of very high SPLs. If you can high-pass them with a sub, that'll be much easier on the amp. 

Several have made accurate observations.  However, too much talk about how loud.  What about how the Mahi Mahi sound?!  I've run them for years and tube rolled them to their pinnacle performance.  They've run Merlins, SA, Epos 11, Reference 3A and more.

The EL-84 is really fast and provides solid controlled bass.  These are about quality not quantity.  Acoustic bass is very well defined, vocals are open and instrument attack is immediate.

And besides, can we all not listen to well done production at a lower level?  Oft times volume is used to compensate for poorly done reproduction.  These allow for very satisfactory listening at sane levels and will not produce insane levels unless you pair with horns.