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

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.

The Manley’s use EL84 which are not exactly big power tubes. In fact, they run in triode mode at 20watts (UL 40w) which will sound more nuanced and you may prefer it that way which limits you to a fairly low power set-up. If you do like to rock out, you may prefer a KT88/150 based amp.

The difference between 150w and 70w is ~3db. This is a very slight difference in volume. The a21 amp is 250w, so dropping to 40w is a difference of ~8db, and if the OP's Gold 300 are the newer 4 ohm model, the a21 is 400w, then making it a 10db difference. These larger drops in power have the potential to make a much more noticeable difference in volume.

With over 25 years experience working in the audio industry, I have been exposed to many more amp/speaker combinations than most people. A common problem has always been underpowering a speaker, not so much for volume, but for speaker control. This is likely why MA recommends a 100w minimum. So, what kind of problem? Depending on the music, a significant loss of dynamic range could be the result. Why not contact MA for their opinion?