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

Absolutely agree with testpilot's comment and actually more information is needed to even make an educated guess as to whether you might have problems, room size, music, listening volume, and more speaker info. While searching for MA Gold 300, two different speakers show up on the MA website, one being discontinued. The current Gold 300 is a 4 ohm speaker and will likely measure 87db like the PL300, also rated at 90db, did in a review. Since MA recommends a minimum of 100w on either of the Gold 300's leads me to believe there is something going on with the impedance curve resulting in a speaker that would not be friendly with lower power tubes.

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.

No matter what anyone says, 40 watt tube approximates 100 watt SS. The size of the room is the dominant factor in this equation.

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.

For instance, what about damping factor?  While I'm not a nut on the subject I will say that a 40W tube amp w/ a DF of 3 will not sound anywhere near the same as a 100W SS amp Class A/AB w/ DF of 1,000.  Speaker Q is fixed by the design and the the same speaker will be driven very differently with amps of varying DF.  That is not to say that both amps may not sound good (which is the decision of the listener) but they definitely will not sound the same.

I had a 150 watt SS amp in my listening room, and now I have 70 watt mono-blocks; when it comes to volume, there is no difference.

Many people have said the same thing; I earned a living as an electronics technician, so please believe me, I know all the formulas.

BTW, they definitely do not sound the same; the tube mono-blocks sound much better.