OK... here's a difference I didn't notice right away...
I'm plugging my headphones into my controller, and without touching the volume knob, and playing the exact same song at the exact same volume level, the Sennheiser headphones are not as loud as my Audio Technica's. It's not a big difference, but it's noticeable. I have plenty of headroom remaining, so I can turn the volume knob up higher, I just was surprised at the difference.
Steve-O was kind enough to post the specs of the HD558's, but I can't compare them (the specs) to my AT's because I don't know the model number, nor do I have the original box or literature. However, next time we're debating about budget gear versus high-priced gear, this is a good example of how a lower priced item can compare to a much more expensive one. The Audio Technica's are pretty close to the Sennheiser's, but not quite as good. But the retail price difference was pretty significant.