I found the difference wasn't subtle at all. I have a bunch of MP3s that I made before I figured out how to get normalization working on the PC. In many cases, the difference was as much as 10dB.

Also, maybe I'm crazy, but it appears that in 2.0b11, Hijack is able to apply volume adjustment to the radio, whereas 1.03 with VolAdj only seemed to work on MP3 playback.

Anyway, those few extra dB of difference were just enough that I'm no longer seriously looking to put some kind of pre-amp between the Empeg and the factory amp in my car. (Although, the original factory stereo did get significantly louder than the Empeg ever gets.)