^ P5 has five power supplies - one per channel. One cord feeds three channels, the other feeds two plus the control circuit (whole amp turns off if this is one disconnected). Connecting the two surrounds to this side can help distribute the overall load if this cord is connected to the same circuit as the Sub2 since two surrounds normally don't work as hard as three fronts, likely not even close with your speaker models.
Two 15A circuits are needed if sustained full-output use from all channels is expected, but you'd have be playing considerably louder than at present. If it's a 20A circuit, it may be enough for the entire P5 plus another major component other than the Sub2.
(as the AVM 60 thread natives may be getting restless, please contact tech support if additional info is needed)
The most important noise floor is in your head. Always remember to protect your hearing.