I noticed that you are using an Oppo 971. I was testing out some DVD-R's made for work of a series of training sessions, and while I was testing the discs on all of my players I tried to reproduce your problem with all three of my players (Oppo, a Panasonic DVD recorder, and a Yamaha S1500). In each case, swapping a CD out for a DVD worked without needing to switch inputs. You might want to re-visit the settings on the DVD input - are you using upsample or something similar for your CD listening?