With 15W tube amp, which speakers to use?

Single driver speakers are always efficient due to the absence of power absorbing crossover network. The drawback with most of them is limited bandwidth. Various manufacturers have found ways to stretch the laws of physics to compensate for this to varying degrees. Size and budget will be more limiting for you than number and variety of options. I like Zu.

I use Zu Druids with a 2x15W amp (KT88s), they will fill even a large room with ease. Next to the usual suspects from Klipsch also watch out for WLM Divas with their more refined character.

DCM TFE-200. I heard these demo'd at HE2007 in NYC, run off the Jolida integrated with 15 watts/channel. Truely impressive, and this was a larger meeting room, not one of the tiny guest rooms. The sound was smooth, dynamic, extended, with a very large soundstage. They're cheap, too. www.dcmspeakers.com

Any high(sh) efficiency single driver speaker like Cardersound Madisons.

I use Madisons on a 12wpc SE amp and can drive them to very high volume levels without a hint of distress to my amp.

Regards
Paul

Omega and Reference 3A should be on your list.