XSP-1 is a pre-amp, correct?
So if it were to enhance stereo sound, it would mean the Marantz was introducing distortion which it should not do.
A pre-amp does not color sound, at least it should not.
That is what makes a good pre-amp, it adds no sound, just passing the signal.
So, unless the Marantz is a problem, the answer is, do not buy the Emotiva expecting any enhancement.