Many of the Studio displays were ADC. Are you sure that other port is really DVI and not ADC?
My 2004 MDD vid card has one DVI and one ADC connector. I have it connected to a Viewsonic display from about 2007 with both VGA and DVI ports. When I connect the DVI port directly to the DVI port on the G4, the native res looks good but it too small to read (old, tired eyes). If I change the desktop res to make things a little bigger, the image looks like carp--fuzzy and possibly some distortion.
If I use the DVI-to-VGA adapter on the computer and connect it to the monitor. the image is shape and proper at any support desktop res I select.
You mileage may vary.