Joost,
Nice system.
You are right about the combination of the two. Passive treatment is a good place to start. Here's a couple DIY links you may find useful
Steven P. Helm: DIY Bass Traps
This one has a video link
Acoustics Forum • View topic - Studiotips SuperChunk
These traps will help gobble up the low frequency mush that's hanging out in your corners (Standing waves). Then you fine tune speaker position.
It's a starting point.
Good Luck
Jack