Sorry to hear of the trouble. I set up a 250 DD in December 2010. I built the stand myself out of wood. IMO, I think the top of the stand needs the plywood to hold the stand square. Marineland builds their stands that way. Given the length, width and weight it seems the stand (built with wood) would twist without the top board placing significant strain on the joints and bottom glass. My bottom glass isn't supported by the stand but it has stayed square because of the top plywood. Fortunately, I haven't had any problems after a little over a year.
It's hard to tell but it appears the stand is 3/4" plywood with 1" support pieces. I would think a top plate would be needed to keep the stand from twisting.