With 15W tube amp, which speakers to use?

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.

Duke's Audiokinesis Jazz Modules should work well with your amp -- 92 db and 12 ohm impedance, price new is just within your range -- there is a sale through AudioCircle. I just got a pair this week, and I am a happy camper! The dynamics are amazing. Too bad I have to go out of town for a week.