Usb or Optical

You could buy a used Asus Xonar Essence STX on ebay for anywhere from $50 to $250.  There's an STX II card for $295 if you want the latest version.  These have an RCA S/PDIF output that is better than the optical on your motherboard.  Drivers are available on the Asus website.  However, it's not as good as the Pink Faun S/PDIF.

I would invest in a JCAT USB card ($495), though it’s probably more than you want to spend. Alternative, the Matrix Element H USB card ($329) or the SOTM tX-USBexp USB 3.0 card ($350) might be options. That being said, it’s hard to say whether the motherboard USB output would be better or worse than the motherboard optical output. You would just have to test to see.

If you are using an AMD processor, then look into a Pink Faun S/PDIF card ($360). That will likely be better than a USB card, since you are not looking for DSD.

The EVGA Nu audio card does NOT have an RCA S/PDIF output. That’s what you really want. EVGA has an optical, but we are trying to avoid that interface because RCA coax will always sound better. For S/PDIF output, the Asus Xonar Essence STX will be better.

If you want to get rid of your D300 dac and use a computer sound card for analog output connecting directly to your Little Dot tube amp, then the EVGA Nu audio card will likely be better than the Asus Xonar Essence STX.

I don’t know how the EVGA Nu audio card will compare to your SMSL D300 dac. It will likely sound different in some ways (I could not tell you which would be better).

Maybe the motherboard is limited to 24/48 on the optical?  No idea.  Have you checked the Sound configuration in Control Panel to see if 96.0 kHz is checked in the "Supported Formats" tab?

Try going to "Advanced" tab in your sound configuration in Control Panel and setting the Default Format to "2 channel, 24 bit, 192000 Hz (Studio Quality)".  Then try playing a youtube video and see if the DAC is receiving 24/192.  Alternatively, you can choose another default such as 24/96.  If this works, it could be a limitation of the Amazon player software.

The Windows 7 and Windows 10 sound device system pretty much takes over any audio from an application.  I don't like the way they did this because it over-rides and re-samples/converts the original music data to whatever you set as "default" in the sound control panel.

The Amazon Music app for the PC may not be written to bypass these Windows control panel "Default Format" settings.  Many apps just send the sound file to the internal Windows WMD driver and just let Windows take over and handle the interface with the hardware.  JRiver is one of the few media players that is able to bypass Windows and actually send the music data in original sampling rate (but it mostly only plays files).

So an SPDIF card would be pretty much the last choice.

I don’t actually completely agree with this. USB is required for DSD. However, USB is a data packet interface. It is true that an upgraded USB audio card will help things, but this is not necessarily superior to S/PDIF card. It really depends how good the USB-to-i2s section is in your DAC.

I had an opportunity to compare USB vs S/PDIF on two different DACS. The music server was a $20,000 Pink Faun 2.16x with both USB and S/PDIF AES/EBU output cards. Both output cards had the OCXO clock upgrades.

When using my friend’s $16,000 DAC, the USB connection just did not sound good. It had a hard solid-state edge and the bass was weak. The DAC actually sounded amazing when using a AES/EBU cable, but it was very apparent that the USB connection sounded like crap.

Now when I put in my modified LKS MH-DA004 dac, the USB vs S/PDIF was a lot closer. The USB input on the LKS dac was significantly better than my friend’s $16k dac. It actually sounded pretty good, but it still was not as good as the balanced SPDIF input (AES/EBU).

You may not agree, but your reply indicate you don't understand the reasoning (despite the fact that i laid it out).  USB is asynchronous. It simply reads bits into abuffer and is then reclocekd with a high quality clock. 

You may be theoretically correct for your point of view, but like I said, it all depends on how good the USB-to-i2s receiver section is on the DAC.  In every scenario I have tested, the S/PDIF connection always sounded better than USB.  In some cases, the USB was just horrible (yes, even on a $16,000 DAC!).

for audiomike33, I can definitely see how USB would be the superior outcome here because he is comparing to an optical output from a computer motherboard.  Once you get into the really nice S/PDIF transports, such as the Pink Faun PCI card, things become a lot more difficult.

@audiomike33  - you can continue to use your current USB output if you are happy with it.  Upgrading to a Pink Faun S/PDIF card will improve the result.  Adding an external linear power supply for the Pink Faun will further improve the result (though I doubt if you are going to spend that kind of money).