Funny story: About ten years ago I had the idea to make my own "mini-beast" that would use many J309s in a head amp (not for ES headphones, though). I never got real happy with my simulations, though, and the thought of testing and grading dozens of transistors didn't sound fun. The J309 was probably a poor choice anyway, and they have pretty much disappeared. It was sort of the go-to JFET when I was in college, probably just to suit the comfort level of my professors who had used them.
Discussion of Wrenchone's mod has been given it's own thread, here - https://www.diyaudio.com/community/threads/wrenchones-mosfet-headphone-design.412500/