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

I’m considering changing form parasound a21 to Manley Mahi monoblocks (40 watt tube) to go with my Monitor Audio gold 300s.

the speakers (90 dB sensitivity) say they want minimum 100 wpc.  

What kind of problems (any?) will I run into?

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?

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)