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.

What about egva sound card it's better than the Asus.

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 I'll look into those other sound cards I had the EVGA before and it sounds really good but I think my smsl d300 is going to smoke that EVGA sound card.

In fact I think the smsl d300 is going to be any sound card you can put in your computer.