What are best speakers for low powered tube amps?

Do check out the Verity Audio Tamino and Fidelio. I believe both would be within your budget. I heard the Fidelio sing with Nagra 50W tube monoblocks. As well, the Tamino was easily driven by a relatively expensive low power Arcam SS amp.

Cain & Cain Single Horn Ben Loudspeakers - around $5500-$7000 new. You've gotta step up to something like Avantgarde Duos ($19k-new)to be significantly better.

Look here:

http://www.welbornelabs.com/recomendspeaks.htm

Silverline Sonata MK.lll. Nothing better under $10,000.

I've been through many speakers and am very happily living with with Exemplar Horns - 103db 8ohm (www.exeplaraudio.com)
If you can live with the size (there's always something!) and aesthetic they are excellent. They actually integrate into my realworld environment much better than I imagined...but still you've got to be about the music first.

For your budget, you could go with Cain&Cain BEN doubles (unless Mr. Cain has fallen victim to the rampant greed infecting the high-end currently. At the last CES, the doubles were around $8,000/pr).
IMO, the Cain speakers simply trounce the Avantgardes in every way muscially important to me.

My friend is using the Referemce 3a L'Integral Nouveau's
on his Jadis Orchestra amp. At first he bought the Reference 3A Royal virtuosos. The Royal virtuosos were nice monitors but they lacked bass. The L'Integral Nouveau's are the floor standing version of the Royal virtuosos. There 92 DB and about 42 inches. They sound amazing. They have a much fuller sound and the bass is superior then the Royal virtuosos.
The L'Integral Nouveau's sound very articulate. There one of my favorite sounding speakers. There's a pic of them on Reference 3A's website. www.reference3a.com

Although I don't have much (any) experience with them aside from a very bried listen, I understand that Living Voice speakers are fantastic with tube gear. Worth a look.

I'm surprised no one has mentioned the Coincident speaker line. They are dynamic speakers built with low powered tubes in mind with a number of models you can find new (and used) at your price point. I would also recommend Silverlines. I have the Sonata 2's which (at 95 db and 8 ohms) are exceptional with low powered tubes and also well within your budget. Go listen. Good luck.

Both of the speakers offerd from Zu cable, The Druid2 & The Definition can be driven by as little as 3 watts. Super detailed speakers. I have owned both, the Definition has 4 powered 10" drivers on the back for the low bass & 2 - 10" & a super tweeter on the front. No Crossovers, just High & Low pass filters on the tweeter & the low bass drivers on the back.

Both of these speakrs are great, but the Druid is not full range, it only goes down to 40Hz. The Definition is rated at full range 20Hz to 20KHz.