Different Amplifiers Changing the Width of Soundstage

Subtle changes in the distortion and the slewing of the transients and micro transients. Additionally, with one channel vs the other (as an extra potential aspect).

We hear via transient positive leading edge of signals only. the ear only hears approximately 10% of the signal.

The rest, the other 90% of the signal...the ear ignores, it does not process or 'acquire' it, by design.

Sounds like really high crosstalk?

Phase shift can also cause this issue. Along with the others mentioned (assuming the recording used for this is the same).

Do amp designers voice the phase shift? 

Regarding distortion:

The low feedback amp has the wider (I can hear the other speaker) and more micro-detailed sound when standing in front on one speaker. 

The Class D amp (very low distortion but less micro-detail with smoothing effect) is the one where I can stand in front of one speaker and not hear the other one.  

Do amp designers voice the phase shift? 

You avoid phase shift either by having very wide bandwidth (in excess of 100KHz) or by having at least 35dB of feedback at 20KHz. The latter is very difficult to do BTW.