Yep, what Viper GTS said.
I'm only using the Sennheiser HD495s, which have half the impedance of your headphones, but I still found the line out on a soundcard insufficient.
HeadRoom seems to think that the 570s can be driven by a portable player, but I didn't even find the 495s driven by a Nomad II to have the right volume levels.
For What It's Worth, I was and still am using an SBLive. Before I had an headphone amp, cranking all the volume controls to max did make it fairly loud, but really harsh and a bit unbalanced on different frequencies.
I haven't heard of the Audigy having any particular capabilities. Does he have the Platinum version? I think the livedrive for it might have a headphone jack that could POSSIBLY be amplified. I don't know for certain.