I'm not familiar with what a theatre show might be, at least in terms of musicians...are you playing with other performers? Is there a PA that gets your signal and the amp/speaker is just for you, or is it the only thing that the audience hears as well?
Most people seem to use powered speakers over amps as they (can) sound better for piano and the instruments you mention...for organ, electric piano certain amps work well (motion sound, even guitar amps for electric piano). These are just generalities, I'm sure there's examples of anything working well if tweaked!
In a band setting, I'm a new fan of in-ear monitoring. This would only be an option if the audience is hearing you through a PA, and really is a solution for a particular fix: loud stage volume, especially when next to a rock drummer and marshall-stack guitarist
I have been beyond thrilled at how much more clearly I can hear myself and everyone else in my mix. And perhaps more importanly, my ears are not ringing at the end of the night as they cut out around 30 decibels overall. It doesn't sound like these would be needed for a theatre setting but I still mention it. It may be possible that some kind of headphone monitoring might work for you provided you can hear other performers (if there are any), and of course again if you are using a PA.