Last Friday, received and placed 1 QRD each at the reflection point on front wall behind Apogee Slant 8s (ported hybrid with dynamic woofer and 40" ribbon).
WOW! pretty much sums up my reaction. I've been happy with my system's ability in reproducing rhythmic aspects of music and the immediacy of a singer in the room. Adding the QRDs not only leaves those qualities intact (even slightly improving vocal intelligibility), but they make everything else sound materially better. Sound stage has widened a bit on each side and sense of depth is increased. The instruments in between have been brought up in the mix a bit, sound more realistic and are better spread across the stage. There is an improvement in low level detail (processing on voices, background vocals, decay in instruments). In general, I think the QRDs have helped turn my small room into maybe a medium (or at least a 'big' small) sized room.
I've avoided diffusion mostly because there is a double door on the front wall. So, I have to move the left QRD in order to exit the room. Inconvenient, but I've gotten over it. I could add 2 more QRDs, but would want to hollow one out slightly to fit over the door knobs.
In my case the QRDs have made a remarkable improvement, especially considering the cost of less than $300.
The room is 13.5' x 15' x 9' and is pretty well treated with Real Traps of various sorts.
Rest of system is Wadia 22 transport, Sonic Frontiers SFD-2 MK I (Parts Connexion SE+) DAC, ARC LS 27 pre & Bryston 14B SST2 Pro. Cables are AT&T optical, MIT Magnum MA, 18' MIT Magnum MA (med impedance) & MIT MH-750 biwire.
Listen mostly to indie pop / rock / folk (lately this would be stuff like The National, Meiko, Young Galaxy, Death Cab For Cutie, Sarah Jaffe, Marnie Stern, Sea Pony, Radio Dept, etc).