Different Amplifiers Changing the Width of Soundstage

With one amp, I can stand directly in front of the left speaker and still hear the right speaker. 

With another amp I have, I can stand directly in front of the left speaker and it's like I can't hear the right speaker. 

Does anyone know why the different amps are behaving so differently to the same speakers?

If you have a mono recording, or used a splitter to put the same preamp channel into both amp channels, then one could measure whether the amplifier is outputting a different signal on one channel or the other.

 

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). 

I am not even sure I know what ^this^ Is supposed to mean. What are micro transients and and what is being slewed? Voltage, phase, or what?