Ok, I have the US iPod Classic 120 in my hands.

The difference in sound volume is impressive. I took the Empeg Sirrus test Tone, In Phase, and uploaded on both the EU and the US iPod.

I then connected the earplug output to the mic input of my laptop. I started the Soundcard Oszilloscope mentioned above in this thread. I set it up as follows:
Oscilloscope Tab
Aplitude 0.3
Synch Ch 1&2
Time(Sec) 10m
Channel mode Single

Results:

EU Max V Eff: 337m both left and right channels
US Max V Eff: 463m both left and right channels

I repeated the above test for 8 times.
The difference is remarkable.

By just listening at the two iPods, the difference is just as big. I cannot bear the volume of the US iPod at full throttle, while as yuo know the EU iPod is just high.

Also, I hear more low frequencies and have a better stereo perception.

So, now I am off to upload US firmware on EU iPod. Let's what happens.