• Imaging, placement and orientation

There are a lot of things that can cause a little (or a lot) of excessive sibilance. Of course, the speaker, it's tweeter and network are the obvious ones. The DE250 is a little hotter than the PSD2002, so sometimes people forget to change tweeter resistors when upgrading, which causes a bit too much treble. Other causes are lively rooms and source material, things like that. If you've stayed with the plans, used the right components, you can rule out the loudspeakers. They're properly matched. That leaves the room, setup and source material.

In your case, I'd say you've found your answer. By having the speakers setup the way you have, you've reduced early reflections and improved imaging at the same time. This configuration works best in most cases, making the most natural sound.