I initially was blown away by the sound stage, but after a bit of listening and then going back to my HD600's, I realized they really are almost completely lacking in bass. This is well explained by the FR.
I rebuilt it just to see if more tension would fix it, since at this point I have no clue what's wrong. It did not help the FR to have more tension. Although they are somewhat different, in the grand scheme, they're both largely qualitatively the same in that they have rolloff below 1.5khz.
The elvamide is working well, it's just a more resistive coating than others and it takes a while for them to charge up. I don't mind this, I was just checking if perhaps that was somehow contributing to the frequency rolloff. It also had no effect on the FR with regard to the rolloff problem.
So, basically I have no idea what's wrong. The first electrostatic headphones I made had almost flat frequency response right down to <50hz (unfortunately these headphones are long gone and scrapped, I don't really keep old ones together), then every new model I've built have had more and more rolloff below 1.5khz. Unless I figure out what I must be doing wrong, pretty soon I'll just be building electrostatic high pass filters!