Vintage Synth Repair

Hello. I'm the owner of a seventy three key Rhodes Mark I stage piano 
and want more from the output level. However, I don't want to damage 
the amp I'm using. (Fender Princeton Chorus) I saw something in the 
service manual that refered to installing a transformer to boost the 
output level. Please respond if you have experience with this 
modification or if you have any other ideas. Thanks a lot.

Sincerely
Joshua