Using with my 6xx. Im a bit dissapointed as my hifime dac is cleaner sounding. The LQ is a bit grainy in comparison to using the hifime dac alone. Although you do get more power and a richer sound signiture with the spark. However the trade off in transparency does not bode well for using the spark.
I have about 25 hrs on the spark so it may get better with more usage.
Btw there is no comparison to my CTH (using Genalex Gold Lion) which excels in every catagory over the Spark and the hifime DAC.
I think that its laughable to compare the Spark to the CTH.