You really don't need an external amp to drive the house music system, but it is advisable since most impedance matching devices run around 6 ohms when running multiple pairs of speakers.
Most higher end brands of receivers can power these systems but it's not about watts, it's about amperage. That's why they call the amplifiers, not wattifiers.
What do you plan on listening to for the house speakers? if you plan on doing something like Pandora or another streaming source, I would recommend something like the Sonos feeding an external amp. The nice feature is that you can have your music on your computer and the Sonos can see your music and stream it. You can even add an Apple Airport express to it and use it as an Airplay device.
If, you want to listen to cable or sat music channels supplied through the cable or sat box, fm , or cd then you can route them through the the receiver to zone 2.
I don't ever recommend using impedance matching volume controls without an impedance matching speaker selector because the byproduct is heat at the volume control.