However, you will probably get optimal performance by making USB cable shorter and S/PDIF cable longer.
jh 
Hi Jim, why do you think so?
In my case I have about 35 feet from my PC to my audio system.
USB cables don’t come any longer then 16feet and over that they say to use data buffers. I bought a buffered USB cable and a non buffered USB extension to add to a 16’ cable. There was no noticeable difference between the cables and combinations of them.
However, its been my experience that digital coaxial cables have a giant impact on the sound and the shorter the better and I mean short, like a couple of feet!
I don’t think the USB cable really makes any difference in sound. I tried a thin, cheap’ass USB cable and a big beefy (heavily shielded) one and there was NO difference what so ever – very strange. I mean USB is transmitted through RF with the squeezebox and people are just gaga over them. Try that with Analong line level or SPDIF, I don’t; think you could without serious degradation meanwhile with works perfectly with RF and over standard RJ45. My device is USB I2C DAC.
I would think using the longest USB cable with the absolute shortest coaxial would be prime.