I2S will not sound better than a asynchronous USB DAC. Asynchronous USB sets the PC into a slave device (PC isn't the word clock like in adaptive mode)-> via buffering the USB DAC has the control over the timing of the audio. So there are no jitter problems if you have an asynchronous USB DAC. The HRT Streamer and Wavelength DACs are all asynchronous USB DACs!
Read more @ http://www.computeraudiophile......dio-Primer
I don't think it's the best way to use the onboard I2S connection if you want a high quality device. Better use a USB DSP to transform the signal into I2S or S/PDIF like the
miniSTREAMER miniDSP connect the DSP to twisted pear audio's Buffalo DAC or to a Digelaty DAC 2010 (both use Sabre ES9018). The problem is you need at least one separate power supply.
I think the easiest way is to use a asynchronous USB DAC - to enhance the USB power supply you can implement with little soldering one of those Panasonic fm boards like this http://www.thel-audioworld.de/.....ng/STA.htm