Cross-talk and distortion, chief soundstage contributors...

And, don't forget about harmonic distortion.-Especially odd and even ordered harmonics, which the human ear can distinguish and play a role in how the sound of the amplifier is perceived.
Bob

Interchannel crosstalk:  the TOTAL amount  two channels are crossed.

vs

Channel separation: where they are NOT crossed at all.

vs

Crosstalk: one source bleeding into another source normally through a 
preamp.

Answer my own questions then. Words matter!!!

OP wants to know about interchannel crosstalk, not channel separation
again differential vs non differential amps, talking about amps, and sound stage, right..

Fully differential preamp and amp will give the best results.  That is the quest OPs on, right, wrong? Eliminate distortion via complete channel separation.

What does a cart have to do with that, all carts have channel separation issues. Still the best when it's right.

Regards

A more relevant amplifier characteristic is the ability to fully and believably render the complex harmonic structure of different instruments. When these are not done well then the instrument never does sound truly believable and so it is impossible for the ear/brain to ever tell you its really there. Yes the famous Fremer line, there's more there there. 

This is another one we don't have a specification or measurement for but people can hear it, they know when its there and when its better and when its worse, and the amps that do the best at this also do the best at throwing a believable sound stage.

Answer my own questions then. Words matter!!!

If this was directed towards me, I am not obligated to reply to anyone.  I often chose to reply to people who are nice and use complete sentences and make cogent arguments which leave open the possibility of a misunderstanding.

Best,
E