I don't know either, I have only heard it from multiple owners of the HD280 that the low frequency responce in itself...in terms of real life quantity and presence is increased a good amount with a good amp past "baseline" (ipods etc)
I use an amp to rectify some problems with some of my headphones as well. To cool off the SRH440's harshness, and give it some more oomph and to add "weight" to my AKG K422's
To bring out the 'true' headphone characteristics, especially for headphones with wild impedance swings (and low to moderate overall impedance), I think a near-zero output impedance amp should be best.
To that effect, I'm not sure how well would a soundcard alone would work for the OP.
Typical "headphone out" can have some output impedance. Even the Xonar STX, that has a "dedicated" headphone amp, has a 10-11 Ohm output impedance.