Question about set amps and low sensitivity spkrs

No real mistake was made. You wanted, and found an SET amp. You already knew you'd need more efficient speakers to go with it. You'll be able to listen to what you have until you get the new speakers. As long as you are building your system based on accurate information, you'll achieve your goal, regardless of the purchasing order.

The short answer is yes you can use the SET with your B&W without doing damage (though your tubes will work harder than they would with a different speaker, but that isn't damge, just wearing out tube life), but the specs suggest a match far from ideal. Like others have said, the sensitivity per se is not too much of a problem at most listening levels, though measurements I saw in audioXpress suggest the real sensitivity is really closer to 88db. The real problem is that speaker has very large phase angles (large swings in impedance) which are not so hot for hot for SETs or tubes generally (why many B&W speakers don't always match well with many tube amps). So, you should be fine for now, but if you love the amp, you do need a different speaker to hear it at its best. With 18 watts I would look for 92db minimum and smooth 8ohm or higher impedance curves.

Coincident, Devore, Classic Audio Reproductions, Zu Audio, Audiokinesis.

These companies all make speakers that will work with your SET amp.

No harm in trying both 4 and 8 ohm taps and going with whichever ones sounds best. I have found 8ohm taps to sound better than 4 ohm taps with 4 ohm speakers- but it is a case by case basis.

-Rob

Having owned B & W loudspeakers before with similar sensitivity and impedence, 90db/watt and 4 ohm, I can say that mine craved a good deal of power. If mine were like yours this impedence value may drop to 3.6 ohms, as it was listed in the manual- NOT the sort of load I would want to pair with an SET.