@DavidMahler : I have finally buy a new HD800. For the moment I don't have headphone amp just a USB external sound card. The first thing I notice it's the coherency of the sound, it's not bass - medium - treble separatly, it's the full spectrum together. It puts some air between the instrument or the voice and the instrument, I like this.
I have owning a HE500, it have more lush and tactile sound, but the bass was a bit bloated and don't have that coherence between the frequencies.
I wait for a great amp to judge the HD800.
!! You'll certainly benefit sound wise from a headphone amp over a usb sound card. You're facing another fine journey getting those headphones to perform the way they should. He500 owner too.... a lot of money spent on phones compared to the source, which is arguably just as important. And just as much fun finding a good one.