What output voltage were you measuring that HP distortion at, or at what dBSPL, was it 106dBSPL (you just mention that it clips there, but nothing about what dBSPL it was measured at)
Here's a your plot showing the amp clipping at 1kHz at 114dBSPL.
I would encourage you to really measure the voltage distortion at high frequencies though, with an audio analzyer or scope, at high votlage, to rule that out.
For the frequency response, a lot of estats tend to do that at high frequencies, they are all roughly the same type of design, an incredibly thin mylar, and open back. You can pad roll too try and it does change that response in the highs. I also chose to keep my design very open for transparency. They sound great to me and most people, so I don't want to change anything about it honestly. If I tried to add padding to reduce resonances, it will sound muffled (mylar is super thin and the electrostatic force is weak) therefore it would loose a lot of SPEED that estats are known for. Again you probably look at that and think that's not objective enough for you and that's fine. Finally for those that don't understand the world of estats, the biggest complaint with them is that they do not have a lot of bass, which is what my main goal is with mine and why people like them.... Now I know you guys don't care about the "tech", and just the F response against the Harman target, but just explaining so you know where I, and many others are coming from. Finally, Kevin Gilmore didn't design a bunch of electrostatic amps for the Hive, it was for STAX.. so please understand there is this aspect to the community about the importance of amplifiers.
Please I'd like to see more electrostatics on here so at least I have something to benchmark against. Measure the Perun, R1 Conquest, Voce, SR009 and others.