How to make small room sound bigger

@rixthetrick, In this or his case diffusion behind the listener is completely wasted I would imagine. 

IMO absorption may be the preferred way to reduce the low range band levels and with direct radiated energy diffusion and absorption will be useful (needed). 

Low Freq will be the most difficult to treat and being the highest energy level the most important. As noted above placement is wide open for good results.

The Binary panels linked - look like a fine product and a quick glance at the Abfuser performance chart shows the effective range.

Congratulations on the QRD panels - well done!

@rixthetrick, lol - in the pic the L/R channels placement looks like 10-12 feet apart.

It's not easy (for me) to imagine what the sound field is like, but it does speak to the idea of the original post's inquiry of ' making room sound bigger ' which in fact treatment should do. 

What I would like to make clear is that the direct sound from the speaker is bounced off the back wall (if not effectively diffused), and then makes its way back to the diffusor behind the speakers.