Question about set amps and low sensitivity spkrs

I agree with both Mr N and mr T: You won't hurt the SET at all listening to your B&W's. You may even be surprised by the levels it can achieve(though I wouldn't get too ambitious). Just don't try to match the SPLs the Arcam delivered. It'll start to sound bad before you get that far anyway.

Thanks for all the informative, helpful responses, very much appreciated. Well, the amp has both 4 and 8 ohm taps...the speakers are rated at 8 ohm, so I would still use the 4? Or should I sue the 8 ohm amp taps? It's funny everyone tells me I'm going about this the wrong way, by buying the amp first! I just figured I could choose the amp I liked and try to match speakers to it, but this seems to be a beginners mistake! Thanks again!

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.