Best speakers for low power tube amp

Soliloquy,Silverline,Coincident and Equation 25s all are excellent with low powered tubes!

Check out Reimer speakers. Very good for low-powered tube amps.

Given that I don't much care for the sound of horn-based speaker systems, a number of the possible choices drop out of the equation. I have, however, heard several of the higher-priced models in the Coincident Technology speaker line driven by low-powered SET's and the like, and I was quite impressed. The President and founder of Coincident (one of several excellent Canadian speaker manufacturers), Israel Blume, has a strong commitment to building efficient speakers with top-notch cabinetry. For more info about the Coincident line, check their Web site at:
http://www.coincidentspeaker.com/

Really depends on budget. I think all the above are excellent choices.
If your budget will allow I would not hesitate to recommend the Audio Note AN/E sec spz speakers that a fellow a-goner has listed (don't know the gent so I have no personal interest...just know the speaker). This speaker is 94 db with a benign impedance curve and all silver wired with Alnico magnets. It is a two way with incredible refinement and musicality.
The price is definately right on this pair and I can't think of anything that would compete at this used price.

The caution is...this speaker requires quality throughout your playback chain to get to it's capabilities.

FWIW I like my Alon Lotus's that I'm driving with Cary 330B 11 watt monoblocs. Sensitivity is 91 db but the woofer has a 14 ohm impedance making it easy to drive. I can certainly hit uncomfortably loud levels in my 12x18 room. But as others have said, there's a world of options, the best advice is to listen to everything you can before you buy.

Consider some single driver speakers also.

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

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.

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.