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

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.

@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.