The interaction between the onboard audio/soundcard and the headphones can create an uneven freq response:
http://www.afrotechmods.com/reallycheap/soundcard/sennheiser.htm
And the graph from HeadRoom that shows the HD 600 impedance:
http://graphs.headphone.com/graphCompare.php?graphType=7&graphID[]=573
So basically, with your integrated audio, you may have a situation where there is less bass being reproduced than should be ("low end roll off"), because of the load they present to the line driver. This doesn't mean the source has an explicitly non-linear response, it just means it's mismatched/ill-suited for what it's driving. You can demonstrate something similar by plugging your headphones into a device that expects a line-level input as its load (do pick something with a volume control); it will work but it will not work "well."
It's not really a question of power; the HD 600 can get ear-splittingly loud with very minimal input (most any dynamic headphones can). It's more a question of whether or not that "loud" is letting you get the proper FR from the transducer you've purchased (the HD 600 in this case), or if it's being screwed with by some other factor (like the impedance mismatch). A headphone amplifier is one option, and I'm in favor of that kind of purchase for desktop use; I don't have a favorite device to suggest - the Creek OBH-11 and Firestone Cute were popular a few years ago. Neither should be terribly expensive. Fiio seems to be fairly popular today, I'm unfamiliar.
Quote:
Originally Posted by
AndreiKuznetsov 
What does "low-end roll-off" mean? As things get wonky at lower volumes? Please explain to the n00b audiophile D:
