The diaphragms have a very limited Xmax. They get non linear easily creating distortion at higher frequencies never mind the doppler effect. The ESLs become even more effortless and higher sound pressure levels stay perfectly controlled and relaxed. Without subwoofers the ESLs will start sounding stressed at higher volumes. This is certainly the case with Acoustats. I can not see why it would be different with Sound Labs speakers.Roger used to make the B1 subwoofer which was an ESL panel that, if you had a pair of them, sat between the A1s creating a wall of ESL from one side of the room to the other. Several of my customers have had this sort of setup, using an electronic crossover so there was no bass excursion on the mains. I seem to recall the crossover to be about 200Hz. By getting the bass off of the mains they got more transparent.
IMO there were tradeoffs- the problem being that the Sound Labs and MA-2s driving them were a pretty transparent system for which the crossover at the time was no match. Roger began to mess with the spacing on the sub, and the later versions seemed to get harder to drive on account of the spacing between the diaphragm and stator being increased (inverse square law...).
I like to play my system at some pretty high levels too. If you want the entrance of the gods into Valhalla to sound right, you can't pussyfoot around with the volume; Wagner scored a serious brass section at that point and its not the sort of thing you sleep to :)
Of course you understand that I spend far more time playing our own amps than I do most solid state amps, and by comparison my comments are correct. I concede there is a perspective issue afoot!