As a monoral signal has no separation, by the same token, it would seem to me that an amp with better channel separation would throw a wider soundstage than one with poor separation.
BUT, perhaps a bit of out-of-phase channel crosstalk could make the image wider?
Also, one would want both channels to be/track as identically as possible, to keep the soundstage from shifting about. If a channel lacks what the other has, it can't really produce mono for a proper center image at that frequency/volume. This would apply anywhere along the audio chain.