How powerful is a Wavac MD-300B

Pani,
The problem is that the power rating doesn't tell you much. There is a
hierarchy that exists in the ranks of 300b amplifiers , based of quality of
construction, power supply and transformers. This is why one will drive a
particular speaker well and another ( same stated power rating will
struggle). The two WAVAC SET 300b amps may have the same power on a
test bench but likely sound "very" different and vary in their drive
capability.

I don't agree with the blanket statements made about these amp's
limitations, you must consider which 300b amp is in question for reasons
already explained. My 8 watt SET drives my 94db/ 14 ohm speakers easily.
Isochronism is right, at some point you simply must get these amps on
your tannoy and find out for yourself.

I have a 300b Jolida amp that sounds much louder than the 8 watts would have you believe. It might be because of the structure of the harmonic distortion. Our ears/brain judge volume in part by certain distortions. So I appreciate that it "sounds" louder without actully being at a higher SPL. I figure it is saving my hearing!

Speaker/amp matching is, IMO, much more critical with a 300b SET so I also recommend that you try the 300b amp with your speakers if possible before purchase or get confirmation from others that are using your speakers with a 300b amp.

It often comes down to the transformers and the overall caliber of the circuit. There are some SET amps that are very powerful and others not so much. I don't know about the Wavac, I would have thought that they would be in the high power bracket yet others say no, so...

When I had them hooked up the Sophia Electric 300B mono blocks easily drove my Tannoy Kennsingtons to very high levels in a large 16' X 30' room with 10 foot ceilings. Using the deHavilland UltraVerve as pre.

I just found a review by Jack Roberts at Dagogo of the EC-300B, where he begins by stating that he also had previously lived with the MD-300B for quite some time prior. I found it strange that he did not give a comparison between the two.
I then found his previous review of the MD-300B. He basically says about the same for the both.
At three times of a price difference... there should be a big difference...