Importance of wall behind listener?

My sweetspot backs up to the rear wall. This is paramount for good bass slam and visceral impact. Move the seating position further into the room and bass starts to lean out. Even a few inches further into the room makes a difference. You can see my setup in system pics...

It is interesting to hear the different opinions on this subject. But in the end it all comes down to the room your hi-fi is located in, the orientation of hi-fi in said room, and how much money one is willing and able to spend on acoustic treatments. I actually put my 8'x11' rugs on my walls when I was still mid-fi. It was a relatively inexpensive way to aesthetically and acoustically treat a room with 10' flat ceiling's and a lot of wall space. Also, the room my hi-fi is located in was the best choice in my home, as I have a 9' pool table in my other large room. When my house was being built, I even took into consideration which of my two large rooms I would use for my hi-fi in regards to ambient noise levels outside my home during late night loud/listening sessions. My gear had to be orientated in this room a certain way due to a door in the room that leads out to the backyard. Hence, IMR (in my room), I really have no choice but to put the listening position against the rear wall. But, in playing with that seating position both fore and aft a few inches, I've noticed that it profoundly affects bass slam. I will say that there is a more enveloping soundstage with the seating postion pulled out from the rear wall, but at the expense of bass slam. And I'll admit, I'm a bit of a (tight) bass junkie. I wish I would have been into high-end when the house was being built-there are so many things that could have been done in preperation for a proper musical experience (i.e., many dedicated circuits, insulation, wallboard, etc). All that being said, and as deep as I am into high-end now, I would completely design my next (if there is a next) home around my hi-fi. ;-)