I have been trying various types of music through various laptops of late. I noticed that my AppleTV was producing much better results than music streamed straight from my laptops (both MAC and Windows), when feeding an external DAC. I have been playing with 96kz/24 bit music form HDTracks, Apple lossless and various FLAC sources.
I was talking with the Benchmark Media folks about upgrading my DAC1 to the new DAC1 pre. While talking on the phone, we got off topic and started to discuss computers as a digital source.
He pointed me to their online advice area. This is an incredible help. I cannot believe how much of an improvement in the quality of playback through an external DAC. There isn't much focus on jitter, as the Benchmark DAC1 essentially makes it a non-issue.
Hope this helps you as much as it helped me.