That was our experience in the Durango: The OEM-1 was OK, but it just won't crank massively loud like the original factory stereo.

The reason for all of this is that the factory Infinity amp expects low-speaker-level inputs instead of line-level inputs. The OEM-1 is supposed to amplify the inputs up to a level that the factory amp expects. It just doesn't do quite enough for the Infinity amp. I wonder if there's a way to "mod" the OEM-1 to get louder?