<font color='#000080'>VERY common problem. Nobody pays attention to other people's line level/voltage regulation settings. There's no default scale or methodology, so this is a problem we'll have to live with.
This is also why I so highly praised Onkyo/Integra for their IntelliVolume circuit. You can raise and lower the starting point for each of the inputs on their receivers/processors to combat this problem. In your case, you woudl boost your cable or lower the DVD "starting" volume using IntelliVolume. More manufacturers ne3ed to clue into this. Alas, they may be too busy adding auto room-EQ and adding 3 and 4 zone outputs...
The real frustrating part is when you start to use "digital" cable boxes that need to be connected via analogue AND digital because only the upper channels are truly digital. There is a 10dB difference in analogue and digital channels on my SA box, and unless you want to set up a discreet input on your receiver for each, IntelliVolume can't even help this problem - yet.
[Edit: I may have switched your cable/DVD volume needs, but you get the idea]</font>