Usb or Optical

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.

Weird thing is I got my smsl d300 hooked up through USB and I'm playing Amazon music I can't get above 24/48 when I check on the song I'm playing it does 24/96 but it says my device is only capable of 2448 that ain't right can't figure out what's going on even when I go somewhere to play a higher rate flac file the screen on the DAC never goes above 48.

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?