AH!
I forgot a little info: You won't hear anything from "LSP IR Mono" as
long as you won't choose a "source" in the middle buttom of the GUI. I
choose "left" for all four channels - I don't know, what this is all
about, but as I already wrote: it won't work, without.
Also, I don't know, if "LSP IR Mono" will compensate, anyway, but
theoretically you should add the time to "pre delay" that was cut off
with "headcut". So, f.e., if your IR file starts accurately with the
impulse and you use 20 ms of headcut, I had good results adding 20 ms to
"pre delay", too.
Greets!
Mitsch