@ozzy you should also look into ATS Acoustic's QRD diffusers. They're similar to GIK's, but they're a better value (lower cost per square foot of wall coverage). They will also custom make them in any height you want for a minimal upcharge.
Another option are GIK's polycylindrical diffusers (they call them Polyfusors). They will give you a little bit of bass trapping since they're filled with insulation. I just installed a pair of these behind my Maggies.
Have you measured the room? You may wan to start with bass traps. I just installed floor to ceiling traps in three of my corners and the result was jaw dropping. Bass traps should not dull the sound if you get the type that reflect mids and highs. Getting the bass decay times under control is the first order of business imo.