My favorite speaker with an OTL are Kharma's, high effeciency and high enough impedance on most of there models.
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Sound Lab electrostatics. These present the higher impedance load that OTL's crave without the dips characteristic of dynamic speakers with fancy crossover mechanisms. For 4 ohm or other dynamic speakers with impedance fluctuations below 6'ish ohms, the use of an autotransformer (Paul Speltz Zero, Atmasphere's own or equal)is highly recommended - no, its mandatory!
In general, OTL's like high impedance loudspeakers of fairly high efficiency. Eight ohms is usually okay, but many "8 ohm" speakers dip well below 8 ohms in the upper bass or at crossover frequencies.
Also, since OTL's have a high output impedance, they aren't as good at controlling the bottom end if the woofer has a high Qms. On the other hand, low Qms woofers don't rely on the amplifier as much for controlling the woofer's motion, so they work better with OTL amps. Unfortunately, this information is seldom published by loudspeaker system manufacturers.
PHY full-range drivers work very well with OTL amps, as do most full-range drivers (the honest 8-ohm Omega line comes do mind, as does the Cain & Cain line and Lowther-based systems). The Classic Audio Reproductions T-5 works quite well with even the smallest Atma-Sphere OTL amps, and I would expect the high impedance Coincidents to work very well also. Among electrostats, it's true that a powerful (100 watt or more) OTL amp will drive the big Sound Labs, but with a less powerful OTL amp the Quads (ideally the original ESL's - the "57's") would probably be a better match. Disclaimer - I'm a PHY and Omega and Sound Lab dealer.
Best of luck in your quest!
According to the late Harvey Rosenberg, President of New York Audio Laboratories, the OTLs were designed with Quad 63's in mind. I used to be a NYAL dealer and found that the OTLs mate well with most good (high impedance) loudspeakers. If your listening tastes run along the lines of light jazz or classical and your listening room is on the cozy (read, small-ish) side, then the Stax F-81s with OTLs are a match made in heaven. One of the two most memorable experiences I've had in audio was listening to Oscar Peterson through this combination of magical components (tho' the 63s do quite a competent job as well).
Before purchasing a used pair of OTLs, however, be sure to verify that the amps properly bias and balance. The 6 output valves should be replaced with matched sets (if/when needed) and these amplifiers can act very strange when kitted out with unmatched power valves. Also, if you're comfortable with a soldering gun, I'd highly recommend replacing the 10 or so square, red capacitors with exact value matches of the best current "magic" caps you can find. This single modification is relatively easy to perform and will yield a noticable improvement in the amp's overall transparency and speed.
p.s. If you're interested, you can read some of Harvey Rosenberg's last articles on audio at: http://TriumphPC.com/psst!/jun01