In many of the stores I've seen very capable arrangers plugged into mono keyboard amplifiers, utterly ruining the sound........

I admit, many of today's stereo whizz-bang arrangers have not had their samples checked for excellent mono compatibility, and most of their effects sections make use of phase differences to 'widen' the apparent soundstage when in stereo, resulting in an apparent change in effects depth when heard in mono.........

It just leads me to NEVER play in mono if I can possibly help it. If they are designed that way, that's the way they should be played. Only if we scream in a loud enough voice at the manufacturers to double check their mono compatibility will they pay attention, and then it won't matter what you are plugged into.........