Just another thought... if you're using a wireless transmitter for the accordian why not pick up a radio scanner (they're quite small and cheap nowadays) tune it to your transmitter frequency and use it with an earphone?
You could also use the same idea but put a mic/guitar radio transmitter into one of the monitor outputs on the main mixer desk.
Re - earphones I've found sony Hi-Fi ones as good as any for monitoring (if it's good enough to listen to me CDs it's good enough to listen to me live!)