Well, let's see what crossfeed does, alright. As I understand (and in veeeeeery layman terms), it eliminates the way that sounds either come from the left or the right side, which can sound weird on headphones, thereby reducing the "stereo" effect and taking everything "to the middle", so to speak, reducing the spatial locatability/soundstage. At least, that's how I always thought it worked. (Again, I'm obviously not a tech person, so my apologies if the above sounds overly crude.)
Edit: I think
@ubs28 explained it way more concisely and clearly than myself in the post above. And yeah, my music also sounds more centered, direct and upfront with crossfeed at max on my Pioneer SE-Master1. But perhaps we can agree that the effect is quite subtle all in all.