I just received my Explorer from Todd The Vinyl Junkie in the US. Not dead certain that it's the latest firmware but given the date and assurances on Todd's site I presume it is.
So far I've tried the Sennheiser IE80's (analytical) and the B&W P5's (mellow).
I'm using BitPerfect with iTunes on a mac.
I would give the sound quality an A+.
It does a great job with 96/24 tracks from HDTracks.com and AAC files from the iTunes store and my own vinyl rips with a Sound Devices 702. It sounds better than the headphone playback in the Sound Devices itself.
The only drawback is that it's not loud enough for some of my tracks. I have the volume all the way up in bit perfect and it's not really loud enough for Led Zeppelin.
Cheers,
Sam

G61R, DSP5200, DSP5200HC, DSP3100, HD621, Oppo UDP203, HDFury Vrooom, MC200. Happy!