If we think about it in it’s simplist terms, a deep, wide soundstage requires a stereo signal. The more "mono" the signal is, the more it will "move to the center" narrowing the soundstage assuming the channels are in phase. If the channel interactions are out of phase, the soundstage gets more "nebulous" as separation is lost.
So, it would seem that an amp that has poor channel separation would not produce as nice and wide a soundstage as an amp that has better separation. And of course, anything (e.g., interchannel distortion) that affects both channels similarly is also in mono so hurts the soundstage presentation.