Best speakers for low power tube amp

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.

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.

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.

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/