If you want to, or have to, do your own AVR-890 firmware update, it is possible.
Although Denon did not include a proper RS232 connector, there is a TTL RS232 interface on the CPU board (CP801). Note that this is a TTL RS232 interface, so you can't just directly connect it to the RS232 connector on your PC. I got an interface cable from here: http://www.superdroidrobots.com/shop/item.aspx/rs232-3-5v-ttl-type-2-serial-converter-cable/337/
, but I still had to make an adapter cable to connect the four pins of this cable to the smaller pin spacing of CP801.
As far as I can tell, the silkscreen on the CPU board is incorrect for all the pins on this connector. I found the top one to be +5V, second to be ground, 3rd is receive, and 4th is transmit.