It's best to have the listner away from the back wall as Shadorne mentions. But if you cant then putting the listener right against a treated back wall can be a very close 2nd best. When right up against the wall the reflections are not delayed enough to cause blurring.
I use GIK products for treating the wall directly behind me (aka the back wall).
I use GIK products for treating the wall directly behind me (aka the back wall).