Sometimes, not always, being way back in the room and running 90+ degrees means you're literally pointing at the side walls, which can add a lot of indirect sound, and if the room is boomy to begin with, it can make matters worse. I usually narrow the angle at that point, especially once I start to know the room and that it does that. You give up some stereo image, but you get back more of your direct sound (thinking PA example here). I will sometimes even go XY from the back with a narrow angle, giving up all hope of a decent image, but it can still come out better depending on the room, and at least it's not mono (I mean you do get some spaciousness). Also, I know going wider can add to the boom, but sometimes when I need the angle to be more narrow (so I'm not pointed at walls), I then space 'em wider in the nmae of getting some of the image back. Anyway, just a few thoughts...