I saw a post on the BitPerfect FaceBook page stating that the software drivers for the W4S DAC-2 will not work with OS X Mavericks. Unfortunately I saw this after going ahead and installing Mavericks along with the new iTunes (it does come with Mavericks if you don't already have it.)
I'm glad I use Carbon Copy Cloner, and I did a complete backup beforehand, so I was able to restore everything without incident. I just thought I'd post this in case others with similar gear are thinking about Mavericks. The W4S web site doesn't mention anything about this yet; of course Mavericks was only released a few days ago. H
Just for those who are curious, the problem I experienced after the temporary upgrade was no sound at all. I checked Audio-Midi setup, and everything was OK there, but the sound pane in System Preferences would show the mac's line out selected for sound output and not the W4S. If I changed it, and opened up the pref pane again later, it would have reverted to the other output again; selecting the W4S for sound output just wouldn't hold.
I'm glad I checked the Facebook page. I'm back on Mountain Lion now, and everything is perfect. I am using the newest iTunes, though, without incident. Hopefully Wyred4Sound will have updated drivers soon.