Usb or Optical

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

I don’t actually completely agree with this.

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.  If you don't have a high quality clock, you dont have a HQ DAC.

SPDIF, including Toslink, demands that the source clock and the sink derive timing.  Not good and the source clock is likely quite poor anyway

People love re-clockers. Yet a USB interface IS a reclocker. It must be.

First thing is to determine best input on dac, generally going to be usb with most. Once you determine this, you then optimize the compatible output on server or streamer. Optimization means both inputs and outputs optimized, otherwise not truly optimized. For usb out that means JCAT, Pink Faun usb card or external usb renderer. I'll repeat, very few servers with optimized usb. For example, Innuous Zen certainly seems popular, Innuous also makes Innuous PhoenixUSB, seems Innuous is very aware of their server's usb liabilities.

Wow quite a debate going on here.  I know my Dac will do 24/192 from optical, so above poster is wrong.  You do need USB to get dsd out of the Dac.  Dsd has never really taken off and probally never will so im not worried about it really.

I built my computer myself and did have a Evga soundcard in it and it sounded pretty good.  I thought going to a desktop Dac would improve sound quality and features.  I took a chance on the SMSL D300 cause it has a completly different Dac chip.  I dont like ESS dac sound, METALIC comes to mind, i do like Bur Brown but the only only company who uses one is IFI.  Anyways this Dac sounds so so clean and analog sounding, great depth, good buy.

I had no idea i could by another card and put in my computer and run the usb out of that for optimal sound, how much better will it sound with this usb card ? im guessing not $400 worth.

I am using the trial for Audirvana - from my laptop USB to back of the DAC. The improvement of sound quality is significant. 

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).