When you say six zone, do you want each zone to play a separate source at the same time, or just be able to play one or two sources across six different zones? If you are mostly using the system for Radio and for Pandora and want Smart Phone control, I would look at Sonos. Right now I have one Sonos Amp that is running seven different "zones". Each zone has it's own impedance matching volume control that I just leave in the "off" position unless I'm listening in that zone. A Sonos Amp will set you back about $500 but it has built in Internet Radio (all of my local stations are on there), it can play Pandora, Spotify, Siruis and all off your local music as well. Another nice feature that was already mentioned is being able to scale the system up easily. I plan on adding a few more Sonos Amps to my system in the near future so I can power a few more zones and/or have different sources playing in different zones.
I haven't looked at all of the other systems out there, but when I decided to build my system I wanted to be able to control everything from my phone, tablet or PC without having to go to a keypad to change tracks, select sources and so on. Sonos happened to be the best choice for what I wanted. It's Good luck!!