If loaded slew rate at 20khz limits the output voltage, the amplifier is poorly defined. A lot of the discussion points that have come up recently have been fairly binary issues (it either is slew rate limited, or it isn't... it either does have enough vrms to get desired SPL, or it doesn't... etc). Any amplifier that's not a straight up bad design has enough slew rate, and has enough vrms. Without a doubt, all the amps with a good reputation like BHSE, LL, WES, etc. are going to have enough slew rate and enough voltage swing.
So, at that point you can look right past those numbers, because they quite literally don't matter given that there is enough (that is, when comparison shopping, if you came across an amp that said 100vp-p, you should run away from it. But if you are comparing amps with 800vp-p and 1200vp-p, the difference is completely irrelevant and you should treat them both as "yes, has enough voltage"). What then starts to matter is things like harmonic distortion and intermodulation distortion. These sorts of things will be affected by a load, and therefore different electrostatic transducers might affect an amp differently. A BHSE has feedback so headphones barely affect the output, but with something like the Wes the load could affect the output quite a bit.
Edited by dude_500 - 4/1/14 at 9:12am