I've got to be honest. I always thought my Crack with Speedball was missing just a tiny bit of something. My set up is this
Mac Mini > Fiio E17 DAC USB > Crack >Sennehiser HD650
Just for fun I hooked up the Fiio E17 to Optical instead of USB. What an across the board improvement, at least in this setup. I know the jury has always been out on what's better, USB or Toslink SPDIF however I think it varies DAC to DAC, system to system. In this implementation it just works.
In the interest of upping the game a hair more I just ordered a Schiit Bifrost with Optical only and going to play around with that.
What say other users? Optical or USB as your source AS IT RELATES TO THE CRACK ONLY?