I acquired an RX-V671 a few days ago, and have been loving it.
However I've searched around everywhere to try and figure this out, but I can't seem to find a definite answer as to whether the main impedance setting (8 or 6 ohms) also applies to the Zone 2 speakers. The Zone 2 setup faq merely says not to hook them to speakers with an impedance lower than 6 ohms.
So I'm leaning towards thinking this means that it's ok to hook 6-ohm speakers to the Zone 2 terminals even though the only impedance setting available is set to 8 ohms (to match the Main Zone speakers), since separate amps are presumably involved... but I'm leery about jeopardizing the health of this fine system. Especially since on the back there's a box drawn around all the outputs (including the Zone 2 ones), that says if the setting is 8 ohms, then both the front and "other" speakers should be at 8 ohms.
Perhaps this is obvious, but it's my first dedicated receiver, so I have much to learn!