How powerful is a Wavac MD-300B

I listened to that Amp a few years ago but with a 99dB speaker....that was ok.
The 300B does not have the abilities to Drive unsensitive Designs, 93dB is normally not enough for such amps but you should try first.
I listened to Westminster speakers also with 200w, their owners wanted it that way.....

Sensitivity of a given speaker is inly one information, even more important is, what kind of X-over is inside? Some can be a black hole for tube amps, even when speaker sensitivity gives the impression, that low powered amps can be used (Wilson is among such examples). Some tube amps are quite robust and have the ability to drive a wider range of speakers than others (Lamm ML2.1 is such an example but between ok and great there is also a difference) but I never heard a 300B amp in its full glory below 98dB speakers...