definately get a new reciever...denon?
you can use imp. matching volume controls and run all five sets on zone 2
Are you sharing the reciever? are you also using it for a surrond reciever?
or ,depending on what type of speakers your using , you can get a 12 channel amp, you'll have an extra pair but i'm sure you'll find a use for it.
as far as switching, you'll probably have to get into some sort of control system, maybe keypads?
i guess you can use a niles speaker selector between the amp and the speakers ,then you'll be able to switch on and off manually.
a 12ch amp would have line level inputs.