Thanks warpdrv & robbroy for your comments.
I already played with REW but I haven't got any good MIC for the moment.
I tried the one that comes with the Onkyo 875 for the Audessey setup but I guess this is not "an ordinary" MIC and very specific for the Audessey application...
I'll start with the "regular" 80Hz X-over setting and perhaps evaluate up to what point the sound images starts to change...
The DefTechs are now configured as "Full Band" so they get hammered with all the goodies during the movies which makes them jump around (even on their stands) when approaching reference levels
(altough surprisingly they can play very loud en clean!)
I guess I will be already "offloading" them quite some when setting X-over on 80Hz...
Thanks for the input!