On my setup when Atmos is selected the tonality, and soundstage both change, with the front stage moving farther away from the MLP.
Ron, I'm curious to know if you know why that is. To my understanding, the bed channels are not effected by the atmos decoder.
Having a non atmos system, I can't compare directly, but it seems to me that even core 7.1 tracks are quite amazing. Panning is smoother, stage depth is deeper, and everything just seems mo-betta. Those are back end things I'll probably never know about, but the front end is most definitely improved.