In my opinion (which is not unbiased), single-driver speakers are generally limited in clarity and/or articulation when called upon to reproduce large-scale music at high SPLs in a big room. That's just not where they excel.
Again imo, the coherence of a single-driver speaker arises largely from its lack of significant off-axis frequency response glitches which commonly occur in crossover regions. I believe that frequency response glitches (including off-axis ones) are typically much more audible than any phase non-linearities multi-driver speakers may have, which is good news because it is possible to minimize such off-axis glitches and thereby rival the coherence of a single-driver system.
For compatibility with an OTL amp, smoothness of the impedance curve is arguably just as important as having a relatively high impedance. Let me explain: If the speaker is "voiced" to sound smooth on a voltage-source-approximating solid state amp, then its frequency response may be significantly different on a constant-power-approximating OTL amp.
For example, suppose we have an 8-ohm speaker with a 16-ohm peak in the crossover region, and this speaker is voiced for a solid state amp. Into that 16-ohm peak, the solid state amp will be putting out only 1/2 the wattage that it puts out into the 8-ohm portion of the spectrum. Now drive this speaker with an OTL amp that puts out the same wattage into 16 ohms as into 8 ohms, and the speaker is now getting twice the wattage into that 16-ohm peak as its voicing anticipates! So this speaker will sound too forward, too upper-midrangey, and we mistakenly blame the amp because that's what we changed. But the real culprit is poor amplifier/speaker matching.
Having designed speakers with the goal of compatibility with both solid state and OTL amps, my suggestion would be to look for speakers with a relatively even impedance curve. The voicing of such speakers will be much more consistent from one amplifier type to another. Or, alternatively, look for speakers specifically designed for OTL or SET amplifiers; such speakers might not work well on solid state, but that is of academic interest only if you're not going to use solid state.
Finally, impedance peaks in the bass region are virtually inevitable, which can result in overly heavy bass with an OTL amp. Now if the speaker is bass-shy to begin with, the net result can be very good. But in cases where the reduced output impedance of an OTL amp results in excess bass, often the correct bass balance can be restored by lowering the port tuning frequency (either by lengthening the port or reducing its cross-sectional area). A beneficial side effect here would be deeper bass extension than we'd normally get with a solid state amp.
I haven't gone off about the sonic benefits of OTL amps, as presumably you're already sold on that. Take care with speaker matching and you'll have a really sweet-sounding system. Imo and ime.