I tought my Tannoy Legacy Eaton at 89dB sensivity would be too hard for the 300B amp to drive@simna And you were correct. In reality a Push-Pull EL-34 based amp will be barely enough power in most rooms if this efficiency is correct!
SETs are simply out of the question unless your room is the size of a large closet. Something to keep in mind about most SETs: unless they employ feedback (and most don't) the distortion starts to take off once you reach about 20-25% of full power, so if you really want to hear what the amp has to offer (unmeasurable low distortion at low power levels, good for 'inner detail') then you need a loudspeaker with enough efficiency to allow that to happen. A 300b-based SET will thus need a loudspeaker of about 103dB or so in an average sized room.
European rooms on average tend to be smaller than American listening rooms, so you might get away with an SET if your room is really small enough.