Have you tried playing with inverse toe in? In other words put the speakers to point at the side walls at your listening position, then gradually move towards the paralell position until you have no blind spot in the middle of the soundstage.
Side walls are a known source of harshness due to causing early reflections. So I see this as bad advice. Instead, toe in the speakers, possibly to the point that they cross in front of the listening position. Side wall reflections must be controlled else they are well-known to cause harshness. If the tweeter has a wide dispersion characteristic, this can make setup tricky.