High efficiency is critical.

Stick with the Heresy's. I bought a pair to use with my SET amps: Sanei 560A 2A3 and Will Vincent 45. Also my SE 6BG6 custom build. They have sufficient sensitivity to work well even with the SE 45 (1.75 watts). Yes, bass response drops off below 50hz, but for most rock/jazz not a problem. For the rest I have a Carver sub.

You could get Khorns if you have a large enough room with two free corners. Or the Speakerlab version, to save money! Come to think of it, my Speakerlab 7's would probably work well with the Sanei 2A3 amp. They resemble a Heresy with an extra woofer on the bottom - no sub needed! Recommended!

Tekton Double Impact............ 98db efficient

For that budget range, look at speakers from Decware, Omega, and Zu Audio. If your Heresys are first gen, you could upgrade to the IIIs, then take them a bit further with outboard crossovers from ALK.

You could also go with a used pair of 604s.  I do and augment the bottom and I couldn’t be happier. 

Try to find actual test results for sensitivity.  The two Tekton speakers reviewed by Stereophile found that the actual sensitivity of the speakers was abut 6db (!) lower than advertised.   

Altec Valencia's or Alec 19's! I have owned the first and listened to the 19's.

Try to find actual test results for sensitivity. The two Tekton speakers reviewed by Stereophile found that the actual sensitivity of the speakers was abut 6db (!) lower than advertised.

This is not just a failure of Tekton, overrating sensitivity figures is a routine component of specmanship.

However, 92 - 98 dB is merely "high efficiency" for non-horn loaded speakers. :) I think the original poster is looking for 100+, which is strictly in the realm of horn and pro audio.  If you are trying to use ultra-low power (~ 5 W) amps then you have to have this above all.

Best,
E

If you can do some woodworking:
 http://lampizator.eu/SPEAKERS/PROJECTS/P17/Endorphine%20from%20Kingston%20Kitchen.html

Here is the current version: https://www.fikuselectric.com

The new one uses a Dayton Audio PS220-8 8" Point Source Full-Range Neo Driver (FE removes the whizzer), an ATC 18", which I couldn't find, and a very expensive RAAL dipole tweeter. Crossover at 700hz and 3500hz. I used a pair of Altec 515b, Altec 755a and the RAAL. I did buy a pair of the Daytons, but the 755a sounds better. I already had the Altecs. I am powering it with 2, 45 SETs and a DIYaudio Amp Camp Amp single ended 8 watt transistor amp. I was using three 45 amps in a triamp configuration, active crossover, until I discovered that what I thought was cone rubbing on my 755s, was actually the amp freaking out at the 4 or less ohm impedance of the 755s. I don't have a 4 ohm tap on my transformers, and the ACA sounds single ended and doesn't care about the low impedance. The RAAL is the lowest efficiency at 93db/w/m. The others are 102 for the 515 and 106 for the 755. The Daytons are 96. 755a speakers are now obscenely expensive if you can even find a pair. I bought mine 20 years ago.

These speakers have no horn coloration, no box coloration, are flat from 30hz to 30khz, and sound so nice you never want to go to bed at night. If they fall down at anything, they don't image as well as I would like. If you use the RAAL dipole tweeters with the Altec woofers, they work very nicely as a two way, (and image much better), crossed at 1600hz. The Daytons however are only $100 each right now at Parts Express. I bought my RAALs from Solen at $1600 for the pair. These speakers have once again realigned what I thought I knew about hi-fi. My first SET amp was the last (very) major shakeup. I bought the 755s for that amp, and ran them full range, not knowing at the time, or even until last month, they really weren't compatible. My Sound Lab electrostatics, which have been my reference for 30 years, now sit mostly unused.

SETs are worth the effort, good luck in your search.

Definitely Zu Audio if you don’t mind crossover-less super high efficiency drivers (for low power amps). Used Druid MK5 or new Soul Superfly with your budget.  

Omega speakers