There's nothing on controlling the AVR600 in the consumers manual, but this document explains it:
[Link: arcam.co.uk]

It can be controlled through RS232 and IP:
"Controlling theAVR500/AVR600/AV888 via Telnet
It is possible to control theAVR500/AVR600/AV888 via a Telnet session on port 50001 of the IP address of the unit (this can be found in the network
section of the menu). All commands and responses are the same as for RS232 control."

I managed to control it from my desktop PC, so it does work, but just not from my Pronto.