The USB bus can only have a single bus 'master', so when you plug-in a USB stick the device it is connected to is the master, so can read the files off the USB stick - which is a slave device on the bus. In general I would expect an AVR to be a bus master, and the NAS to also be a bus master, so they would not be able to 'talk' to each other. So the external stereo/multi DAC connected to the NAS is seen a USB slave device. Some USB devices (all processor based) are USB On-The-Go and can be either a bus master or a slave on the bus, this uses the USB ID pin on the socket to determine which it is, so cable and software dependent. Again I would not expect either an AVR or NAS to be able to do USB On-The-Go. (NB: I did a couple of circuit designs last year with USB so I had to figure this out). So the way to go is via a NAS and an Ethernet(best)/Wi-Fi connected PC/media player (I'm looking at getting an Intel NUC) and connect via HDMI to the AVR.