Best Digital Interface

Thank you auxinput for some reality. For audio, USB connections are only required if you are using a computer as a music source. In order to use USB data for music it has to be converted into a data stream and clocked. A DAC with a USB input will do this for you. If you do not have a DAC with a USB input then you will require a USB to SDIF converter. I use the Berkley Alpha USB for this. There are various ways to transfer SPDIF streams, they are RCA by 75 Ohm Coax, AES/EBU by balanced 110 ohm cable, BNC by 75 Ohm Coax and finally optical by fiberoptic cable. There is no sonic difference between these formats. It is not music being transferred, it is clocked data, numbers. You do not have music until after the DAC. As for USB being "flawed" ? That is a ridiculous notion. USB does what it is supposed to do wonderfully well. It can transfer data packets much faster then one requires for music data. It is so fast a computer can transfer an entire 10 minute 24/192  song in about 10 seconds. A 10 minute 16/44.1 song would take less than a second.  
Digital audio devices do not need to use a USB connection because they are transferring the data already clocked in real time like playing a record.
They can music data between them in the digital audio formats mentioned above plus HDMI. 
There is no Coax to USB adapter because they carry entirely different signals. There are many sub $1000 streamers that have USB connections. They are called Laptop computers.

I suspect that "Best Digital Interface" and "Best Digital Performance" are apples to oranges.  While we can compare which digital interface is theoretically better, it's the execution in the component that sonically matters.  For instance, for some DACs the USB output sounds best while other DACs....

@kennyc, I think you're right to separate out "Best Digital Interface" and "Best Digital Performance." Best digital interface is only one consideration, while best digital performance will take into account any strengths in our particular streamer outputs and DAC inputs. 

This summer, I was planning to built a Pi and run USB to see if I heard a difference with my Async DAC. From here, I would like to include a reclocking hat and run SPDIF to again see if I heard any difference.  

Apples to oranges apt description. Many if not most dac manufacturers optimize usb input, starting to see I2S implemented more often. I2S theoretically better, implementation is the key. I've found maximizing streaming experience is extremely complex. My streaming equipment chain is almost certainly one of one in the entire world. Perhaps the future may bring more certainty to best interface.

During the 90ties and 00decade, i enjoyed a wadia x32 by spdif with a micromega Duo CD 3.1. Since 2013 my preferred dac is a Mytek DSD 192, using the FW-Output/Oyaide-cable! First, i used a windows-PC/Windows 7 Pro with a FW-PCI-Card.
Since 2020, i've got a streamer by apolitan.de and a PCIe-Card/Chipset by NEC (TI no longer avaiable), on going using the mytek. Comparing to a PS Audio DSD, i'm very satisfied by the mytek/no upsampling.
Frontend by ATMASPHERE and high efficiency speakers (4way-full horn), you can hear every breathing! Are there any other members with similar experiences?