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.

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?