A/B testing my new zero ohm output impedance headphone amp against the amp in my MZ-R909 MD player reveals something astonishing - the Sony MD's headphone amp is no slouch! I have to wonder why portable players get so much stick in this forum. I measure the output impedance of my MD at 2 ohms, and the thing drives the HD580 as loud as I want. Most of the time I have the volume set between 22 and 26, although I have occasionally needed full volume (30/30). Even then it doesn't distort.
I think this performance beats the headphone outputs of most component equipment unless you're trying to drive AKG's or something with voice coils wound out of old toaster elements.
Admittedly the ergonomics of a (Sony) MD recorder used as a general purpose amp are rather poor. You have to go into Rec-Pause mode and also set RecVolume to Manual to prevent any possibility that the automatic gain control will compress the sound. However, if you try this one rainy day and A/B against some more expensive alternatives you might get a pleasant surprise.
Your findings about the relative "sound quality" performance of even cheap portable devices vs. separates and their headphone outputs seems to correlate well with a measurements/listening review published in the Finnish 'Hifi' magazine:
Wow, thanks for the extra info and confirmation Halcyon, that stuff is gold.
The funny thing is, the lower output impedance actually gives you slightly less bass on the HD580, which can sound subjectively less impressive. Previously I had noticed what seemed to be the more "authoritative" and "fuller" sound of a component system headphone jack (as compared to my portable MD) and assumed that the extra power must be the cause. However, now I realize it was an illusion caused by the extra bass. When I listen closely (especially to piano, drums & timpani), the lower midrange and bass are less detailed from a high impedance amplifier.
BTW Halcyon, how about adding a few details to your profile, or is it too early for that yet?