I don't use the DVI inputs on my 990 but I'm pretty sure of one way to do it. Connect the dvd's HDMI (via a HDMI/DVI adapter?) to one of the 990's DVI inputs. The HDMI/DVI from the cable box would connect to the other DVI input on the 990. Then connect the DVI output on the 990 (again with an adapter?) to the TV. This way the 990 does the digital video switching between the dvd and the cable box.
For audio connect the dvd and the cable box's digital audio outputs to the 990 via optical or coax digital inputs; both work equally well. Now the 990 will do the processing and switching of the audio.
In the 990 setup tell it where all the video and audio connections are connected to the back of the 990 and you should be OK. I Have this exact setup except I use the analog video component connections (red, green, blue) versus DVI.
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AvFan
Outlaw 976 | ATI 2005 | M&K 850s Left, Center & Right, Surround-55s, MX-70B Sub | Harmony 950 remote | Panamax 5100 | OPPO UDP-203 | LG OLED55B7