I mostly use my 990 for HT. For 2ch music I use an Aragon D to A processor and Aragon preamp into the same power amp channels that the 990 would feed if I used it for 2 ch music. There is not the difference I had expected in music listening when I purchased the 990. The imaging and clarity is better with stand alone processor and preamp but, if I did not already own the Aragon equipment I would not spend $2-3000 for the difference in the sound. And it is always back to what do you like your music (speakers and electronics) to sound like.
The 990 has (imo)a very good D to A processor in it though and you would have to just try different combination of equipment to see what you like.
I have not tried to send an analog signal from the D to A to the 990 to see how the sound differs if at all from the 990's processor. I am just about finished building a switching unit so I don't have to change interconnects when I change from one piece of equipment to another. It should make comparing the 2 D to A's and the 2 preamps with the same analog signal less difficult. I will let you know what I hear if anyone is interested.
I would be curious to know if most manufacturers of DVD and CD players build their own processors or if many of them use units from the same supplier. I'm sure Gonk and other will probably have information on this.