Hi Brad,
I downloaded the online version of the manual for the MX-600 that can be found here and I didn't see the Sunfire codes in it. And yes P-PRO, then Audio. When I entered the Carver Codes this "stayed" on the display of the remote whereas when I entered the three codes you provided they immediately disappeared with blinking ---, then the "old" code reappeared. I will retry them after a factory reset.
I didn't change the position of Audio.
Thanks a lot Brad and Edmund for your quick reactions.