Difference b/w RPG Skyline and GIK QRD diffusors

I would recommend RPG above the listener (in a low ceiling) and on rear wall behind the listener. I would not recommend RPG on the front wall between the speakers unless you are using Bose or other rearward radiating speakers.

Frankly the majority of frequencies radiating backwards from conventional speakers are the LF - below about 300 Hz and some up to perhaps 600 HZ. So absorption panels or bass traps are most likely the best option there.

An RPG on the front wall may look good and many listeners will be more impressed when they can see and are looking at the RPG skyline but honestly it will be more effective nearer the listener or on the back wall behind the listener where parasitic reflections can muck up your imaging and collapse the sound stage.

Two cents...

Of course, you can easily go overboard with RPG as Elizabeth points out. This studio used TONS of wood and not even ONE piece of wood is the same length.