I use Gumby with BNC for quite a while now. Tried optical and USB before, but kept with BNC after comparing the various inputs back and forth.
I find optical to sound more harsh / brittle / too bright on the highs and USB sound stage more all over the place not so focused. My best way of explaining it, sorry.
The input for the BNC comes from a Neutrik trasformer that convert the AES output of my PC RME AIO sound card to BNC, as there is no AES input on the Gumby.
The Neutrik does (extra) galvanic separation on the PC end.
What makes a BNC a good BNC cable? The impedance and of cause the build quality. I use Procab cable.
Make sure you buy a 75 ohm cable and not 50 ohm version. That is also what the converter does, convert the 110 ohm of AES to 75 BNC as you can see on the image.
For Raspberry Pi BNC out I have recently ordered The Pi Design 502DAC - Pro Audio Shield (tip on this forum). It's shipped, but will take another week to arrive at my home.
http://www.pi2design.com/store/p15/502DAC_-_Pro_Audio_Shield.html
http://www.pi2design.com/uploads/4/8/5/3/48531975/visio-502dac_datasheet_p3.pdf
I have no experience with the DigiOne board. But "10 LDOs, 12 LPF, DC/DC convertors, galvanic isolator, WM8805, flip flops, buffers, NDK oscillators" sounds a lot of Hocus Pocus to me.
Plus the 502DAC also has additional AES-EBU out
I want to go to AES all the way, so one day in future there will be a Yggy to feed it directly AES - AES from PC sound card to DAC.
My dream system is: Yggy - Freya - double Vidar - Quad ESL 63 (or 1912 if I win the lottery)
Freya is in place, Vidar;'s will be ordered as soon as they come available in EU.
Yggy is the next item on the wish list to replace the Gumby.