I think it is safe to assume the USB soundcard that comes with the MMX 2 is crap. In fact, a lot of PC soundcards, USB or PCI, are pretty bad, except the top of the line models.
Running a line out to an amp, even an inexpensive one, is always a superior experience to most mainstream sound card output.
I am hoping they will sound good via line out/optical out paired with an inexpensive, but decent amp.
Consider the price too. The MMX 2 is a 60 dollar headphone modded to have a microphone and packaged with a little usb soundcard. They are not going to compete with headphones that are 2, 3, even 4 times as much. I think the question is...how do they perform for a 99 dollar gaming headset?