@sommerdi Hi! Thank you for your feedback - really useful! I have the IEM plugins, I will experiment.
With my Focusrite Soundcard set to 96KHz and Isadora set to 96KHz with a 128-sample audio buffer I get the least delay - it's very slight, but annoying.
However, when I try this in Reaper with the same audio settings and audio interface the sound is crystal:
Hopefully I'll report back with a solution - possibly the IEM plugin suite.
P.S. The Sennheiser AMBEO plugin seen above is discontinued (Isadora doesn't recognise it) and has been superseded I believe with the dearVR plugin that I'm using in Isadora.
Best Simon