Just something. I quickly ran through the user's manual, but did not encounter a line that says: "Network usable for control", this is what I see:
Quote:

"Te AVR600 and AV888 are ftted with a network audio
client (optional for the AVR500) which is capable of
playing internet radio stations as well as stored music
on a network storage device such as a PC, or on a USB
fash drive."
"Control of the AVR500, AVR600 or the AV888 is
either by front panel control buttons, IR remote control
or RS232 port"

From this I can only see that the receiver can work as a client, so it is able to stream music I doubt it has an embedded web server. Are you sure you can control it over the network? Do you have any additional documentation?

I know I have not answered your question why the tsu does not want to connect (ok maybe I did, hint: client), but I think it is futile to try to connect to something you can't control.