I received the iFi iDac this morning and promptly plugged it into the iUSB and the SB Touch. I downloaded Triode's EDO app and was unable to see the iDac as an option under Output Devices. A simple unplugging and replugging of the iDac and it was picked up by the EDO app and I was able to play music.
But within a few minutes I noticed the music was breaking up. I have been running the Touch Toolbox and I thought I had disabled it but it turns out it was still running. I had to SSH into the Touch and TT -r to restore the Touch to its original configuration. After that it was all good.
But trying to play 192/24 produced a slow motion sound effect. After doing a bit of research I learned that streaming PCM at 192Khz (192/24 FLAC decoded on the server rather than natively on the Touch) was the problem. I changed the "Stream Format" in LMS to FLAC->Native from FLAC->Disabled and that was that, music at 24/192 played perfectly.
As for the SQ, I can say pretty confidently that it is better than the standard Touch by a very large amount but with only around 6 hours on the iDac and with this being my first experience with a Touch/USB dac combo, I will hold off saying any more.
Here's a iPic of the iUSB and the iDac in my current temporary stereo.