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

otherones, depends on what you like listening to and how loud. If you are a stadium  rocker forget it. Jazz, classical and light rock at moderate volumes no problem. Personally, I think you are making a mistake. Save a bit longer and get a more powerful tube amp if you have to have a tube amp. 

Ripple

90db efficiency only tells one part of the story.  How the speaker’s impedance changes over the entire frequency range is very important for tube amp matching. 

I have PrimaLuna mono-blocks that put out 70 watts per side, and 92 DB speakers in a moderate room, and I'm quite satisfied.

They can play much louder than I want them to. I like jazz, as opposed to rock, if that makes a difference.

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

O-10 very happy for you, God Bless !

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.