I don't expect you will get much support for this idea on this forum but since I used my MAC4300v in the same way (what?!? No dedicated listening room?) for at least a decade I will give you my opinions and experience.
Nothing works as well as the dedicated taps and switching for the three pairs of speaker already in your MAC4100. But... you do not have individual volume controls for the pairs (rooms). This is why I have this set up is because I consider my whole house my dedicated listening space and I want to maintain consistent sound as guests and I roam around the entire house. Speaker matching is tricky depending on room size.
I have done much research and tried many solutions including Niles and Russound which were both disappointing. I used separate Niles TVC-1's for each speaker which worked the best because the MAC controlled the switching and master volume and the Niles box just lowered the volume in rooms that were getting over-powered. I tried the SSVC-6 to save space and then hoped the Russound version would be hardier but it was comparable. The hardware for connecting the speakers is pathetic if you are using a heavy gauge for long runs. Its annoying to connect and would lose connections and need to be re-done.
The Niles are all, I believe, rated for 100w and I switched to a supposedly higher rated Russound when I started using a MC2205. Ultimately, and in desperation, I bought a SPECIALTY-AV switcher from ebay which had standard banana plug connectors and supposedly rated for higher watts. I have had no problems with this switcher.
So... in conclusion (finally!), My first recommendation would be to use your MAC4100 switching for the three pairs and and use your preamp out to a separate amp for the other two pairs since you are already considering a separate amp. And if you need the separate volume controls maybe consider the Niles TVC products which I never had a problem with other than the connection hardware. Otherwise... the SPECIALTY-AV switcher has been much more trouble free for me personally.
Nothing works as well as the dedicated taps and switching for the three pairs of speaker already in your MAC4100. But... you do not have individual volume controls for the pairs (rooms). This is why I have this set up is because I consider my whole house my dedicated listening space and I want to maintain consistent sound as guests and I roam around the entire house. Speaker matching is tricky depending on room size.
I have done much research and tried many solutions including Niles and Russound which were both disappointing. I used separate Niles TVC-1's for each speaker which worked the best because the MAC controlled the switching and master volume and the Niles box just lowered the volume in rooms that were getting over-powered. I tried the SSVC-6 to save space and then hoped the Russound version would be hardier but it was comparable. The hardware for connecting the speakers is pathetic if you are using a heavy gauge for long runs. Its annoying to connect and would lose connections and need to be re-done.
The Niles are all, I believe, rated for 100w and I switched to a supposedly higher rated Russound when I started using a MC2205. Ultimately, and in desperation, I bought a SPECIALTY-AV switcher from ebay which had standard banana plug connectors and supposedly rated for higher watts. I have had no problems with this switcher.
So... in conclusion (finally!), My first recommendation would be to use your MAC4100 switching for the three pairs and and use your preamp out to a separate amp for the other two pairs since you are already considering a separate amp. And if you need the separate volume controls maybe consider the Niles TVC products which I never had a problem with other than the connection hardware. Otherwise... the SPECIALTY-AV switcher has been much more trouble free for me personally.