Not to point out the obvious, but two things: You are sure the guy sold you a large KJ, not medium (I don't think they make a medium, so dumb question. 2. Are you sure it's set up right? It looks to me from the pic I see`That the plate setter is not seated into the little L track of the legs of the stainless steel holder. Does the ceramic actually fit into that little L channel, you know the one I'm talking about, you can see it in the sample pic from KJ that you posted. Good luck on that, check out Naked Whiz's site on cooking with the heat deflector in place on a KJ:http://www.nakedwhiz.com/productreviews ... ssic3.html
...also, one other thing, doesn't the guy at Ace have a KJ set up in his store? if your were to take your diffuser stand and stone in there and place it on the floor room model, see if it fits the same?