Hi Ed,

If I were you I would definitely play in STEREO everywhere. All (pro)keyboards are designed to play in stereo and believe me you absolutely should !
I now play the X1 and played many other keyboards on stage and with the right speakers connected the quality of the sound-image improves with at least 100% when stereo. (mono-reverbs ? nah !)
All modern built-in effects are stereo based nowadays e.g. Leslie, chorus, panning, reverb etc.
It is easy to understand the difference between mono and stereo.

MONO : Listen to music while 'closing' one of your ears with an ear-plug or hand.... it is difficult to discriminate the different instruments.

STEREO : listen again with both ears open at the same music.

You should know the difference now.
I do hope you band-members change their minds and are willing to switch very soon.

kind regards,

Roel