Ok, ... I can answer my own OP from 7 years ago. Last year, I swapped out my Ref CD-8 for the Ref CD-9SE. I think the CD-9SE sounds noticeably better than the CD-8, ... but not by a mile. Yes, there seems to be more detail. Bass is a little tighter. Soundstage and imaging are better too. But the CD-8 still does a fine job.
I think the take-away here is that there is only so much juice that can be squeezed out the redbook CD orange. That said, I can see at least one possible improvement; namely: the next round of ARC CDPs that comes off the line has a 3-stage audio circuit like the Ref 6SE and the Ref Phono 3SE. It would also be nice if ARC would build-in a remote on/off button and hour counter on the clicker.
As to a comparison of the CD-6 to the CD-9, I seem to recall that a reviewer from one of the audio magazines made some comments. I can't recall the name of the reviewer or the magazine, but I think I picked up the article from one the reviews collected on the ARC website. Take a look.
BIF