My Adcom CD player only has Digital RCA, so I have to use it connected to the 990. But my Pioneer DV-37 DVD has both Digital RCA and Toslink Optical. With it, I decided to use the Optical since I needed a 2 Meter run.
I would say that my MIT Digital Reference RCA cable ($300 1m) is higher quality than the MIT Optical cable ($150 2m), but I've never used my DVD player to play CDs and do an A/B compare.

I also have a Straight Wire Optical cable from my Motorola DCH HD/DVR box to the 990.