Okay, let's try to keep this simple. There are many ways you could go. First, you need to list what you have to work with. You do have a Xonar D1 so you can obviously, output to your 2.1 speakers. What are you using for headphones?
While the D1 would not be my choice for pure, stereo music listening, there's nothing wrong with it. If you're going to be listening to music primarily, over headphones, then I'd suggest you get a stand-alone amplifier for them, but first we need to know what headphones you're using. If you want to use your 2.1 system to play music most of the time, then all you could really change at this point is the 2.1 system, itself. What's the specific make and model of the 2.1 amplifier? This is very critical information to making sure you have a hardware arrangement that will coalesce.
Matthew