The 0.9degree stepper has arrived and with it the hiss is (mostly) gone! There is still a faint grinding noise at standstill but it's almost as quiet as the old motor was in Stealthchop mode! I still plan to look around for MK3 owners to borrow motors from for testing, until than the conclusion seems to be one of the following:
I got unlucky and received a bad batch of motors, or my motors simply wore out (the printer is three years old after all)
Prusa and Ultimachine have managed to cook up some black magic stabiliser for the Einsy