Modifying guitar amp to run on batteries, switching between center tapped transformer and batteries with only DPDT switch
Without seeing the actual schematic, I'd say that there is not problem in doing that. One aspect I'd like to mention is that you probably don't want to switch the amp while it is on. Also, if there is no bleeder resistor for the filter capacitors (which discharge the caps when the amp is off), I'd add one. This way you can be moderately sure that your switch does not switch currents. Another potential problem is inrush current when you switch the amp on - again, without the schematic or at least a make/model, I don't know if the designers had taken any steps to limit this inrush current that can potentially overload the current capability of the switching contacts.