That does "sound" like impedance mismatch, which is kind of like an electrical incompatibility with the headphones. The usual effect is low volume but it's not that necessarily. A headphone amp would help.
Try planar magnetic headphones, they're very easy to drive with anything and you'll definitely hear more than you have before. They have a very strange and noticeable "plucked" sound though, like things that are supposed to reverb don't.
The very best and easiest to drive headphones are electrostatic earspeakers which, uh, you can't afford and neither can I. But I have some Stax ones from the 60s with absolutely terrible construction that sound great anyway.
Thanks for the tip and for helping me understand what might be causing this! I wondered if the original iPods had more power output or something.
The headphones in question were cheap Koss Porta Pro Classics, and they appear to have a 60 Ohm impedance. That seems somewhat normal, but would that stand out to you?
If you've never tried the Porta Pros, they're the best bang/buck I've ever found for headphones.
Try planar magnetic headphones, they're very easy to drive with anything and you'll definitely hear more than you have before. They have a very strange and noticeable "plucked" sound though, like things that are supposed to reverb don't.
The very best and easiest to drive headphones are electrostatic earspeakers which, uh, you can't afford and neither can I. But I have some Stax ones from the 60s with absolutely terrible construction that sound great anyway.