Is your computer near the receiver?
Your setup probably works fine but it could be improved.
You really want to avoid your computer's headphone jack when you are going into another amplifier. A line out jack would be better, because you are bypassing the small output amplifiers of your soundcard.
Optical is the best choice for multi-channel audio. If you have a pci slot in your computer, you can install a sound card with optical out, or you can get an external sound card with USB. The device you linked looks like it will work, but is probably overkill and more expensive than this
which is simply a usb to optical adapter. USB external sound cards are also $20-30 and have optical out.
Your Denon manual will have instructions on how to access the optical input. On my older RCA receiver, it senses when an optical source is active.
Although if your source audio is only 2 channel, you can also improve your sound quality with a sound card that has a line out as opposed to headphone out, and get a mini jack to RCA adapter, with a mini headphone type jack on one end and 2 rca connectors on the other end, and put that into your Denon.
You actually may not even hear a difference. I would put your computer volume at about 25% and do the rest with your Denon.
Before you buy anything, do a test and put your computer at 100% volume and turn up the Denon to a high volume, to see if you can even detect distortion. If you can, turn the computer down to 50% and repeat. Did the distortion go away? Try 25% and lower. Turn up the Denon louder than you listen to normally. If you can't hear distortion, you probably don't need to buy anything else to improve, unless your audio source is more than 2 channel. If you are streaming movies from the shuttle, then definitely get optical.
On the other hand, I have a setup I use where I have a macbook and plug the headphone out into a soundboard that outputs to 2 very large PA's and a very large sub, enough to fill an entire gym with sound. Even at high volumes it is clear, no matter the volume setting on the macbook.