Where/How can I further improve PC audio quality?

latest DAC with asynchronous inputs. Asynchronous mode is technically most advanced for re-timing and re-clocking. It will deliver the best sound possible. I purchased the ZDAC for $300 last year and the different is huge and never look back.
May be you can further improve Audio quality with high end brand name, however I only believe in technology not pricing.

I assume that you're using the "on board" sound from your computer. That's a big problem. As stated, A USB DAC, without any up sampling/down sampling, will go a long way to improve sound.

Is it that the sound from the computer is not equal to that from the 840c playing CDs and you are happy with the 840c when doing so? One issue is that you are streaming a lower resolution file from Spotify, therefore the signal from the computer is not as good as that from the transport. That said, you might benefit from sending the digital signal from computer via USB and using a USB/SPDIF converter and going coax to the 840c. This way you take advantage of the 840c’s DAC and pay only for the digital conversion device.

I thank all of you for your input. I am not using an "on board" connection, I'm using a PCI card. At this time I am using the highest resolution the card can deliver, I'm going to try experimenting by setting the card to minimum, i.e. 16 bit 44.1Khz and letting the CDP do all of the upsampling work.