Welcome to the forum!
Here is a
link to a comparison between the two units from Denon's website. It appears that there are more differences than the RS-232 and the price. Some that I saw at first glance are power output, connections, and type of Audyssey room correction. However, the specs on Denon's site list RS-232 for the 889, but I don't see a jack in the pictures. I'm downloading the owner's manual now to look through it.
EDIT: Nothing in the owner's manual about RS-232 on the 889, either. Unless there was an update to the hardware, that's a mistake on Denon's spec list. Makes me wonder what else isn't correct. So, maybe that comparison will help, maybe not.

At least it can point you to things to check out.