highly sensitive loudspeakers for single ended poe

Granted, they're quite a bit under your budget, but they may work for you. The GoldenEar Triton Two towers have a rated sensitivity of 91 dB and are only $3K/pair. HOWEVER, they have built-in powered subwoofers and a single pair of speaker cables supplies signal to the whole thing. The most telling thing is that the designer/co-founder, Sandy Gross, co-founder of Polk and Definitive Technology, uses a SET to power his personal pair at home.

When you have 1200 wpc built-in to power the bass, you may find that 91 dB sensitivity for the rest of it will be enough.

Another possibility is the Zu Definition Mk IV. It also has a built-in powered sub, but sensitivity is spec'd at 101 dB and they're priced at $12,500/pair. However, the Definition Mk III is also in production, also has 101 dB rated sensitivity, built-in bass amp, and they're $7500/pair.

It's also my understanding that Zu structures their impedance curves to be friendly with low-powered SET amps.

I found Living Voice speakers to be extremely tube friendly and their sensitivity is 94db.

To get the best results out of an SET, unless its fairly high powered, the speaker should really not be less than about 97 db. Otherwise the amp will be making a lot of distortion, meaning that it will not sound as transparent and will also take on a harsher quality.

Typically efficiencies 10x higher than stated in prior posts (101 db to 109 db) are preferred- with these higher efficiencies the distortion of many SETs becomes unmeasurable.

This is why horn systems like Altec, JBL, Klipsch, Electro Voice and others have held their value so well. But in general, you want either a horn system or a high efficiency single-driver full range speaker like a Lowther, PHY or similar.

Now if you are in a small room the lessor efficiencies may work. As in many cases, YMMV.

I use a 300b SET with my Zu Def4s, and the sound fills my large space without any difficulty. It depends on the tube, the design of the SET, the output power, and many other factors including the room and (in my case) a very long run of speaker cable. If you are interested, you should call Zu to discuss. As I have learned, almost every SET is different and should not be lumped together. I had Def2s and tried 45 and 2a3 tubes before settling on 300b. Others swear by 845s. Other speaker considerations include the soon to be released Zu Druid 5, Audio Note, and Living Voice.

Another vote for Daedalus. I spent a lot of time looking for SET friendly speakers, but avoiding horns. I have'nt found a horn speaker, I could enjoy long term.

The 3 brands I realy enjoyed were Silverline Devore and Daedalus. The latter seemed the best in term of price, appearance and sound quality and my DA-RMa's run just fine with a 20 watt Ayon Spark. I am not sure a 10watt 300B, would be a great long term match.