Originally Posted by WindowsX
All iPods before classic series down to gen3 has the same volume.
That isn't true. I bet all iPods are a tiny bit different. I've found that even my two 5.5G iPods don't have perfectly matched levels. They all require a little bit of adjustment. It isn't easy to do though. It takes careful listening and lots of A/B switching. If you just unplug the amp from one and plug it into the other, it may sound the same, but if you do instantaneous switching from source to source you can hear the slight difference.
A slightly lower volume can make the sound appear to be duller and not as transparent as a slightly louder source. Once the volumes are matched, all the differences in sound fall away.