The following article is written by John H. Darko of Digitalaudioreview.net regarding why USB audio quality varies. Here are a few extracts from the piece.
“Having already tested and written about the Schiit Wyrd, the AudioQuest JitterBug, the UpTone Regen and the Wyred4Sound Recovery, extensive real world experience tells me that USB signal cleaning devices like these can make a noticeable difference to sound quality.”
“the iFi iPurifier 2 connects directly to the DAC’s USB port and cleans up the USB signal after it has left the host device’s USB port and journeyed down the USB cable…..the iFi requires no intervening cable or dongle to make the DAC connection; it plugs directly into the port…..The iPurifier 2 is also an active (powered) device yet there is no external power brick feeding it. Instead, it gets it go juice from the host device (computer/streamer) – in our case the Sonicorbiter SE. The iFi dongle brings greater tonal and micro-dynamic avidity to the Sonore table.”
Thank you John H. Darko and Digital Audio review!
http://www.digitalaudioreview.net/2016/05/gordon-rankin-on-why-usb-audio-quality-varies