That Marantz has Zone 2 output, as long as you are using 5.1, not 7.1. You can use Zone 2 pre-outs to feed the Rotel to drive the whole house speakers. You can connect all the speakers to the SS-6 and the Rotel will probably have enough power to drive them, although I would check to see if it can handle the 5 ohm load from the SS-6. In-wall and outdoor speakers are usually pretty efficient so the Rotel should be OK if it has 100W output. You will probably need analog inputs to use Zone 2. Also, check to see if the speakers are all 8 ohm. That many 4 ohm speakers would definitely be a problem
You can certainly use a simple RCA splitter to separate the signal in order to use 4 channels of the Rotel. There can be some minor impedance issues between the Marantz and the Rotel doing that, but the effect is pretty low and should not be an issue for whole house speakers.
The biggest problem will be volume control. You are almost certainty going to want to be able to change volume for different rooms. You could use impedance balanced speaker selector boxes like the Niles SSVC-6. Make sure such a box has impedance matching. For example, the Monoprice ones do not, I believe.
So, I would check the speaker impedances, the impedance the Rotel will handle and think about volume control.
You can certainly use a simple RCA splitter to separate the signal in order to use 4 channels of the Rotel. There can be some minor impedance issues between the Marantz and the Rotel doing that, but the effect is pretty low and should not be an issue for whole house speakers.
The biggest problem will be volume control. You are almost certainty going to want to be able to change volume for different rooms. You could use impedance balanced speaker selector boxes like the Niles SSVC-6. Make sure such a box has impedance matching. For example, the Monoprice ones do not, I believe.
So, I would check the speaker impedances, the impedance the Rotel will handle and think about volume control.