I'd consider a 2012 Yamaha instead of the RX-A730. The 2013 models don't really add much in new features (MHL, faster HDMI swiching) and you should be able find them cheaper. In particular I'd recommend the HTR-7065 or RX-V773WA (both the same receiver except the later comes bundled with a wireless dongle) as they have more functionality than the RX-A720 and a normally lower price.
The only AV receiver currently on the market capable of playing external digital audio sources in an independent zone 2 is the Yamaha RX-A3020. Some older 2011 and earlier Denons supported optical/coaxial digital audio in zone 2.
However with most AV receivers, Yamaha's included, internal sources like AirPlay count as analogue so you're able to select them in zone 2. Yahama receivers that support Party Mode (HTR-7065, RX-V773, but not RX-A720 or RX-V673) can also play any source, including HDMI or optical/coaxial, in zone 2 when Party Mode is used to play the same source in both the main zone and zone 2.
The Yamaha website should let you do a detailed comparision of the features, but some of the features the RX-V773 has that the RX-V673 doesn't have include two HDMI outs, pre-outs, multi-point YPAO and the already mentioned Party Mode.
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