@ozzy, I like your approach of diffusing rather than absorbing the backwave. Ime the backwave energy of a good full-range dipole can be very beneficial, but you want to preserve its spectral content rather than degrading it by using absorption, which will remove the shorter wavelengths moreso than the longer ones.
When you say that you want "to make the soundstage even larger", that can imply different things. Strong early same-side-wall reflections tend to widen the soundstage, but dipoles have a null to the side so that may not be practical. Greater soundstage depth and/or an increased sense of immersion in the acoustic environment on the recording is imo more likely to be feasible with appropriate frontwall treatment.
I think you said your speakers are about 30" from the side walls. How much distance between the backs of your speakers and the wall behind them? Ime this distance plays a significant role in perception because it strongly influences the time delay between the first arrival sound and the strong onset of reflections.
You mentioned that Clarisys uses a reflector behind their speakers at audio shows. Imo that makes a lot of sense. Have you heard them at audio shows? If so, what did you think? And, what do they suggest for your situation?
I might be able to make some general suggestions based on your responses to these questions, but imo making specific acoustic treatment recommendations is something that I’ve learned to leave to the pros because they can make a MUCH bigger improvement than I can. If you don’t have anyone in mind I recommend Jeff Hedback of Hedback Designed Acoustics, he’s a multi-award-winning studio designer who also does home audio, and he’s still affordable. He works remotely.
Duke
dipole speaker dealer for 25 years, multidirectional speaker manufacturer for 16 years