I've been using the optical in on my C2500 to stream music through an Apple airport. There are light pops that sound like a vinyl crackle that are happening, sometimes a few in a short time and other times there are long pauses between, but they are pretty regular. I thought it could be a tube issue, but when I'm using the MM input, there are no pops, and also I've plugged the optical cable and Airport into a CD input using a little cheap DAC and no pops there either. The music sounds much better through the built-in DAC, just not sure where the pops are coming from. Any thoughts?