Where/How can I further improve PC audio quality?

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.

spotify is MP3. Anything will be better.

re Spotify Premium (from Wikipedia, Spotify entry)...

"Spotify software is proprietary and uses digital rights management (DRM) to prevent unauthorised use. Users who agree to Spotify's Terms and conditions agree to not reverse engineer the application.[87]

Audio streams are in the Vorbis format at q5 (ca. 160 kbit/s),[88] or optional q9 (ca. 320 kbit/s)[89] for Premium subscribers. Spotify has a median playback latency of 265 ms. It amounts to 390 ms without local cache usage.[90]"

(from Wikipedia Vorbis entry)

Vorbis had been shown to perform significantly better than many other lossy audio formats in the past in that it produced smaller files at equivalent or higher quality while retaining computational complexity comparable to other MDCT formats such as AAC or Windows Media Audio.[28][29] However, as of 2014, not many further significant tests had been made.

Listening tests have attempted to find the best quality lossy audio codecs at certain bitrates. Some conclusions made by listening tests:

Low bitrate (fewer than 64 kbit/s): the most recent (2007) public multiformat test at 48 kbit/s showed that aoTuV Vorbis had a better quality than WMA and LC-AAC, the same quality as WMA Professional, and a lower quality than HE-AAC.[30]

Mid to low bitrates (fewer than 128 kbit/s down to 64 kbit/s): private tests in 2005 at 80 kbit/s and 96 kbit/s showed that aoTuV Vorbis had a better quality than other lossy audio formats (LC-AAC, HE-AAC, MP3, MPC, WMA).[31][32]
High bitrates (greater than 128 kbit/s): most people do not hear significant differences. However, trained listeners can often hear significant differences between codecs at identical bitrates, and aoTuV Vorbis performed better than LC-AAC, MP3, and MPC.[33]

I have no business interest in Spotify but am a satisfied Premium subscriber.