I really prefer the sound of the USB input to that of the Firewire. I've given Firewire every chance I could, but it always comes in second to my ears. I'm using the same cable (Audioquest Forest) for both, newest drivers, and same output formats out of JRiver. I expected Firewire to sound better, and used it exclusively for a while, but it has not been the case.
Also, I'm going to register a minority opinion here. I don't really enjoy the headphone out from the Mytek itself. Every time I compare it to the balanced output into my amp (an ECP Audio L2), it falls far short. The bass is anemic from the headphone jack, and the soundstage is vastly smaller. I ran a quick volume-matched ABX (with the help of my lovely wife) and correctly identified the Mytek or the ECP each time, and never preferred the Mytek.
Everything feels constrained out of the headphone out, frequency extension is limited on both ends, and the bass is significantly less textured. A lot of detail is smeared, which really detracts from how holographic the DAC really is.
I ran this test on a pair of JH13s, Grado RS1, and Sennhesier HD800. I heard similar differences on all three, which leads me to believe that it's not an issue of current, but of design. I suspect the headphone out on the Mytek is a bolted on chip amp, and while it uses the same excellent power supply as the rest of the unit, it's not designed to the same standard. This unit is designed as a DAC first, preamp secon, and headamp third. If you can get away with using only the DAC, it's a great unit.