Mate a quick call or email to GME might be the best thing. I have just taken my new 600D out after finding out post professional installation that it will not talk to my raymarine A65 without a software modification. I had GME look at the unit and they have certified it 100% working (in writing) but advised me that the two units were not compatible.
Do you already have the 600D? I am just about to list mine for sale.
Brendan