I have 6 all together.
The difference was more than subtle after installing them. The corner traps helped with low end more than anything else. The panels behind the speakers made the biggest difference, they helped get rid of room echo. Lastly, I placed two above the listening position on the back wall. IMO, they deadened the room too much and I now have them behind my couch, maybe they're helping low end, but doubt it. I want to experiment a little more though, I may place one horizontally between the picture and my equipment rack.
I've also been considering GIK's new diffusers for my back wall.