Best speakers for low power tube amp

There is such a large number of speakers that would sound good, that I don’t know where to begin. Everyone has their opinion as to what would sound the best, and most suggestions made would probably be good ones. Because of what I just stated, I’ll leave you with this website that’s full of great speakers for your situation: http://www.welbornelabs.com/index.html
Search this site, there are dozens of speakers and they would all work very well on your system.

I don't know your budget, but if you're looking for something under $3K then I would highly recommend either Galante Rhapsody (96dB) or Reference de DeCapo i (92bB). Both are extremely easy to drive and 18 watts should be more than enough. Both are excellent speakers which sound gorgeous with SETs. If I were to choose between the two, I would go with the Galantes. I found them a little sweeter than the de Capos, they image better, the midrange has more palpability and they are easier to drive. Actually, I could drive them to almost insane levels with a small 3.5 watts 2A3 based amp, which wasn't enough for the de Capos. With your 18 watts drivability will not be an issue and I'm sure you could be happy with either of these great speakers.

I agree with Markxiii. The Galantes are probably some of the most underrated speakers on the market. They sound heavenly with low powered SETs.

Consider some single driver speakers also.