Are USB-SPDIF's relevant with today's DAC USB technology?

The easy answer for me is, yes. In my system using a headless modified Mac Mini as a music server (Amarra, Audirvana+ and Esoteric HR Music Player). Using Esoteric K-03x and Audioquest Carbon USB cables (2). After extensive listening, which really in retrospect was not required, the SQ improved dramatically when I installed a Bel canto REFLink USB convertor. (Transparent Ref Digital link (OUT) to K-03x).
The USB implementation of the Esoteric K-03x is very good and current. Certainly worthy of the claim that, improvements can be had by achieving complete isolation from your computer. For me its a no brainer. 

Thank you all for the responses. It looks like a mixed field out there (as everything). Based on some of the comments here and other considerations, I went for a NuWave DSD DAC instead of buying an Audiophilleo for my DL3. I also purchased a WireWorld Starlight USB cable.

My consideration is that for similar price, I would gain advances in both USB technologies and DAC technologies. Every review I read comparing the DL3 and original NuWave said that the NuWave is a big improvement, so I am assuming the NuWave DSD is more?

@gocubs999 comment is interesting. I would be surprised the USB of such a device of the K-03x could be improved.. but apparently it could.

Regarding replacing my PC as a player, I've been wanting to build a custom player similar to the Bryston BDP-Pi.
I've already experimented with this a few years ago, with a RPi and HifiBerry which worked quite well. What I want to do is to build a linear PSU and put everything in a custom enclosure. I could connect that with SPDIF from the HifiBerry (like the Bryston) or via USB direct.

Plug your new USB cable into a good converter like the M2Tech or the Berkeley. Then run a good AES cable (I use MIT) from the converter to the new DAC. That will be better than running your USB cable directly into your new DAC.

Thanks @psag but the M2Tech I had was a Hiface that is either BNC or RCA, and the NuWave DAC I got does not have AES inputs so not relevant in my case

@itsikhefez, 
RPi and HifiBerry w/linear PSU will crush most of the computer bandaids being suggested. The Pi family is an amazing thing...nonprofit scientific community at its best, nice that us audio nuts can take advantage of some of that goodwill and product experience. If you haven't check out Daphile software for those. A builder friend of mine uses it with great results.  You obviously have great building skills and that can take you a long way in this hobby! Cheers,
Spencer