Thanks for the question. The new ES will definitely come well after the HE-400i and HE-560. Perhaps later this year but it is more important to make sure it is perfect so that will determine exactly when it is launched.
Not sure about compatibility with Stax amps. Will see what I can find out.
Btw, I have seen a few Head-Fers calling it "Jade II". That is a name someone made up, not one we are using at HiFiMAN. This ES phone will be nothing like the original Jade.
Thanks Peter, you just made my day!!!