Ok, let me see if I understand you. It sounds like you have an amp that is currently configured for 120-volt operation.
You have an outlet that is dedicated 220v (20amp?) You want to reconfigure the amp and plug it into the 220volt outlet.
In addition to changing the taps around inside the amp you will most definitely need to change the cord cap (male adaptor) on the amp cord to match the wall receptacle. Is the receptacle twist lock (15a or 20a?) or does it have two horizontal ports on top with a ground port underneath? (Sleepy eyes)
I would not attempt doing anything without talking to the factory and a good electrician first, one mistake and POOF!!!
IMO it would be easier to re-configure the outlet back to 120-volt than re-configure the amp.
Under no circumstances should you ever increase the breaker size. The breaker was sized to protect the branch circuit. Increasing the breaker size without increasing the wire size is in fact a violation of the National Electrical Code