Subwoofer Pops with Nvidia High Definition Audio

I upgraded my desktop sound system to KEF Q150 speakers, Sumiko S.0 self powered subwoofer, and a NAD 3020v2 amplifier.  I connected the amplifier to the Nvidia High Definition Audio via an audio splitter to use the optical in on the amplifier since it doesn't have an HDMI in.  With the gain more than half way the subwoofer pops every once in a while.  Does anyone know why?

I tried connecting the analog out from the computer 3.5mm jack to the analog ins on the amplifier with a "Y" splitter and I am not having the popping.  Bud the amp supposedly has a nice DSP when it's connected digitally so I want to see if I can find a solution to stop the popping.

Thanks