Xonar DS line out vs headphone out

I have a Xonar DS with LM4562 opamp and the Sennheiser HD558 (80 ohm) and I've been meaning to ask if anyone knows why the following would happen: when plugged into the line-out, the headphones sound dark, with big, wooly and uncontrolled bass, but the sound also seems fuller and the soundstage is good. When I switch them to the headphone out on the front of the case, the bass becomes much, much more controlled, they sound thinner but more accurate, and the soundstage loses width. I actually prefer this sound, because I can't stand boomy bass (HD25s lows are perfect, IMO).

I know the HD558 have a big impedance spike at 80 ohm, and the HP out on the Xonar can go way louder than the line-out (more current?), but I fail to see how this is all connected, as my understanding of amps and impedances and so on is rather limited. Could any of you head-fiers enlighten me? What I would like is either get a new DAC, or an amp, or a soundcard which would make the HD558s shine, because I don't think I've tapped their full potential with this setup. For the record, I don't have other sources at hand, except my Clip+.

Bump?

Thanks, that might be it. The manual claims that one should use the HP out for headphones, as it's linked to a "powerful amplifier". Which amp this might be, I have no idea. Maybe I need to google more.

Oh.. well, I swapped it out long ago with a LM4562. I might sound a little stupid here, but isn't there a difference between amplifier and opamp? Or are they the same thing? Maybe my understanding of what an opamp is is very wrong.