Efficient, affordable speakers for SETs?

The most efficient loudspeaker I know of is the Klipschorn corner horn.   It's the only loudspeaker that will truly play at level for an SET amp.  You can find them used for $6000 and less.  New they retail for about $15,000.  Cosmetics have a big effect on the sale price.  If you're OK with something that hasn't been treated well, you can find them at a substantial discount.

You'll need a room with opposing corners because they only work properly when pushed into a corner.  They use the walls as part of the bass horn.

If you're so dead set on running an SET amp, then the only real answer is the K-horn.  I think it's the tail wagging the dog, but if you have to run an SET amp, then this is where it leads.

I have used more SET and 300b type amplifiers than I can count with SET front horn systems give the best results. I  also built 100s of full-range driver systems most designed for SET load these can work well. But the front horn systems designed for SET use can give you all the best of SET while you get few to none of the limitations. A good SET on large front horns is a system you can use for life and not feel like your sound is obsoleted you will not tire of it and feel the need for constant change. These systems can be played for many hours and they will not cause fatigue like conventional systems tend to do. Sure the extremely high SPL a 300b SET on a front horn system can generate could overload your ears and room but that is because you are approaching SPL that not even the highest power audiophile dynamic loudspeaker designs can achieve no matter the amount of power you fed them. SET+  horns is a get off the merry go round of audio purchasing system something that just lets you enjoy music and life.

With a 10k budget you should consider the Charney Audio Companion with Voxativ drivers. Charney is located in Somerset central NJ. If your close by contact Charney for a demo.  Well worth it. 
http://charneyaudio.com

Emerald Physics 3.0s should be awesome, but...
FYI: as to SETs, OCDHiFi Guy (YouTube) is now importing 120w++ SET from Serbia

The Klipsch Reference Premiere line of loudspeakers are an excellent, low cost option.
Pi Speakers offers various high efficiency speaker kits that are also an excellent, low cost option.