Seems to me that the weak point in that chain would be the PC motherboard. Even if the DAC in there is good, there are lots of opportunities to introduce noise into the chain inside a PC. You may also be, effectively, double amping the signal (once in the PC, and again in the Yorkville). From a signal quality perspective, your best move may be to introduce an outboard DAC that will convert the digital signal from your PC and provide a line-level out to the Yorkville. One example of this is the Schiit Modi which can be had for about $100. I'm assuming that you're happy with the sound coming out of the Yorkville and your headphones.
One potential weakness I haven't really discussed is your source. If you're listening to MP3s encoded at 128, you might not notice much of a difference with an outboard DAC. In fact, it might accentuate some of the harsher elements in the sound. In such a case, you would do better by upgrading your source.
These are just my ideas. Someone else may have other, equally valid suggestions.