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

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.

celtic66, every recording has a "right" volume. And that right volume is multi-factorial. It depends on the type of music, the way it was recorded and mastered, the way your own system is equalized and the way our ears change frequency response with volume. Bass and treble increase with volume (see Fletcher-Munson). A recording that was meant to be listened to at loud volumes will sound dull and lifeless at low volumes. Turn it up and it sounds great but if you tried to play a string quartet that loud the violins would slay your ear drums. Rickie Lee Jones albums were recorded on the bright side and sound best (more natural) at moderate levels. Turn it up and her voice does that violin thing. I have an old Buddy Miles album with some great songs but it always sounded lifeless to me. I hadn't played it probably in decades. At 95-100 dB the record is a knock out. They were probably stoned and had the monitors turned up to the max when they mastered it. 
A word about listening to loud music. Always warm your ears up. In your middle ear you have a muscle called the Stapedius which dampens the Stapes bone keeping it from vibrating as hard against the oval window. Google it and you can see the anatomy. When exposed to loud sounds your brain tightens the Stapedius through the 7th cranial nerve lessening the amplitude the inner ear sees (hears) but it takes a little while to tighten. Don't just go right to 100 dB. Start off at 85 dB and crank it up 5 dB every few minutes till you get where you want. Beyond 100 dB is no man's land. It also takes a while for the Stapedius to let go which is why after a loud concert you can't hear anything for a while.
celtic66 quantity and quality are essentially the same or so closely tied that you can not separate them. The quality is not there unless the recording is played at the right volume which is determined by all those factors above.