George,
> What do you think about on the wall behind the speakers instead of a floor/wall placement? <
Basically, any place you put them will help at some frequency or other. But the front and rear walls, as well as the tops and bottoms of those walls, are a great place to put bass traps because that's the primary direction the waves travel. When someone starts with only a few MiniTraps I always tell them to favor the front and rear, and leave the sides for later if/when they decide to get some more traps.
Also, I recently added yet more MiniTraps in my living room, including nine of the new 2x2 foot size. You really should come by some time and have a look/listen. I'll even play Graceland for you.
--Ethan
