Ok, litte updates.

Two more tests.

1.
iPod Classic 80GB, US version, 2007 vs iPod Classic 120 EU version, 2008 (mine)
Same headsets: Sennheizer CX500
Same Song (Roxanne, The Police), but different rips.
Difference in max volume very very evident. US iPod was unsustainable at max volume. Mine, as you know, just ok.
Low frequencies seemed better on the US one, maybe due to the more power to the earplugs.

2. Ipod Classic 30 GB, EU Version, 2006, vs iPod Classic 120 EU Version, 2008 (mine).
Same earplugs: sennheizer CX500
Same song, same rip.
Identical output to me.