Before you jump in sound absorption I suggest that you consider covering the window with materials that will mimic the absorption/reflectivity of the rest of the wall behind the speakers. What you are looking for, ideally, is uniformity across the wall. If you have localized hot spots or reflections you can treat those with spot treatments. 1'x1' diffusers work great for this purpose.
The problem with "absorption" as a universal treatment is that no material "absorbs" sound uniformly, across all frequencies. You may end up with something worse and less treatable. IMO if you stabilize the highly reflective spot between your speakers you will have a uniform, treatable surface that you can tune or use DSP.
The problem with "absorption" as a universal treatment is that no material "absorbs" sound uniformly, across all frequencies. You may end up with something worse and less treatable. IMO if you stabilize the highly reflective spot between your speakers you will have a uniform, treatable surface that you can tune or use DSP.