[stereoguy]

Providing you have easy to drive phones, what is so bad about just plugging right into the headphone jack when listening on a PC? There seems to be a school of thought that USB dongles/Dacs are better. Initially I thought plugging in a dongle was an improvement, but actually I am not so sure now (I'm guessing alot of my perceived improvement was due to volume differences). In theory doesnt taking the signal out to USB just add noise versus the PC jack? I'll grant ya the Dongle adds some extra volume kick. But I dont think the sound quality is a huge improvement.

 

[NA Blur]

Most PC's have terribly laid out audio topologies.  They are almost always noisy, rolled off in the bass as well as treble, and have difficulty driving more power hungry headphones like the Sennheiser HD 800.  PC's are designed with CPU operation in mind not audio and this have issues with noise emitting components interfering with the audio output.
 
Things to listen for are:
 
1.)  Can you hear random noise in the background with the volume set to zero?
 
2.)  Can you hear random noise with the volume set to 75% full and no music playing?
 
3.)  Can you drive your headphones so they get loud enough?
 
4.)  Do you hear skipping in the audio when you play music?
 
If the answer is yes to any of these then almost certainly a good external DAC and amp will really improve your listening experience.