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

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?

According to Benchmark Media FAQ distance from the speakers matters only at the open space. Inside of medium size room we can assume that 1W (2.83V@8ohm) will produce 90dB loudness with 90dB/W speakers. Each 10-fold increase in power will result in 10dB loudness increase (twice as loud). 100W amp with 90dB/W efficient speakers should produce 110dB. Do you need 110dB? Do your neighbors need it?
100W is 30% louder than 40W.

(Perceived loudness = k^(1/3.5), where k is ratio of power)

@kijanki But that’s the theory. The reality is that all things being equal, more power can bring an authority to the music like a firmer grip and more breathing space for the music dynamics to fill. It is also true, that much of the action, at medium volume, takes place in the first watt. It all depends on whether the OP wants to crank it.

@mijostyn +1.  It may sound OK at low levels, but you will have no headroom, smaller soundstage, weak bass among other issues.  Not having the headroom is the biggest issue for me.  Also, the idea that tube watts are somehow more than SS watts is pure nonsense.  

Forgive my engineering ignorance....but isn’t there also the consideration of how much wattage is actually utilized on average? I’ve generally thought enormous power “on demand” may not necessarily guarantee a firmer grip on the signal - or be needed - if the circuitry of the amp can allow it to peak and sustain well, and your speakers are rated for a decent relative efficiency. Btw, I’m not asserting this idea, more asking here!