The ST woofers are spaced at equal distances from the tweeter, and they will be at equal distance to you if you're listening on the tweeter axis. The two woofers will therefore image at the same point as the tweeter, and they will be in phase with each other. As you move above or below that point, the relative distance of the woofers as perceived at the listening position will change--that's simple geometry. If you're above the woofers, the top woofer will be closer to you than the bottom, and that changes the relative travel time, and hence the relative phase of the woofers. They will start to go out of phase with each other, and this will cause a cancellation in the output in the upper midrange, lower treble. The effect is not dramatic if you are standing up at a normal listening distance, but you will hear the output in that area start to increase and snap into focus as you sit back down with ears on the tweeter axis.