Well, the SongTowers are -3dB at around 15kHz
at 60 degrees off-axis. They're MTM, albeit vertical. Since the SongCenter is voiced to match, I suspect it'll perform in a reasonably similar manner, just with BSC implemented for horizontal orientation.
@Dennis Murphy did you design the crossover for the SongCenter as well?
In any case, I'm struggling to understand what you mean by "sweet spot" in the context of a center channel. There's no 3D sound stage. There's just mono, anchored to the display. It doesn't matter where you sit in your room, the sound is still coming from the display. And, please forgive me for being so blunt, but how can you be so 1% about your center channel while at the same time be satisfied with 10" subs in your HT?