Excellent willemj! The DSPeaker Anti-Mode 2.0 Dual Core comes with a measuring mic, and has a screen that displays the frequency response of the room, room modes often creating huge peaks and nulls in that response, in both the frequency and time domains. The Anti-Mode then creates a corrective signal to counteract the peaks, but nulls are often uncorrectable---you must move the speakers to deal with them.
There are room mode calculators on the web, into which you insert your room's dimensions. The calculator will show you the physical locations of the room's modes/standing waves, and their frequencies. Bass traps can be placed in those mode locations, and you want to avoid putting your speakers at those locations---they energize the modes.
Higher-frequency reflections are a completely different matter, and can be dealt with via diffusion and/or absorption, as suggested above.