There are a few firewire interfaces available. I went with the M Audio Firewire Solo - $200 and awesome for me. It takes a firewire cable from my PC and has an spdif output (into a monarchy upsampler into MF X DAC). Firewire much better for passing audio as it does so in a stream, as oposed to USB which uses packets. USB was never intended to stream audio. It was intended to connect peripherials.
I had extensively auditioned the Ayre with the Esoteric X-03 SE and I felt the Esoteric has better retrieval of low level detail, more air around the instruments and a wider-deeper soundstage. The did both do a great job and didn't sound very different at first, but after about 5 hours of listening, the Esoteric became my preference. I felt the Esoteric brought me into the music as opposed to the Ayer, which brought the presnetation of the music to me. The D-05 had a tad wider and deeper soundstage than the X-03 SE (the dac was hooked up to an X03 SE as a transport). On the Stereophile recording, The Mooch, the Esoteric reproduced the music and the venue itself. I had never experienced this before on a CD. With Chesky's Rebecca Pidgeon, Spanish Harlem, the D 05 was slightly more holographic than the X-03 SE -
I would settle for an X-03 only Dac if there was one (I don't believe there is) which is why I am considering the D05 for my squeezebox (replacing the Benchmark).