I want to use the (speaker muting) headphone output on stage for self-monitoring. Is there any way to modify the amp so that both speaker and headphone works? Perhaps with a jumper? And is there a separate volume control for the headphone out?
No, sorry, there's no way to modify this. Consider that although the RH450 performs like an analog amp, it's digital and 100% software driven. In this light, it's not simply a question of adding more components, but also altering the software to take the changes into effect.