Thanks so much for explaining @gkkpch!
First of all I wanted to thank you for the great job of porting the new Volumio 3 to Odroid.
It came at the right moment while my Hifiberry suddenly decided to start suffering from power/ground(?) issues on the Raspberry 4. The cause is explained elsewhere on this forum and not solvable apparantly. According to Hifiberry at least.
This sparked my interest in the Odroid again, which I was using for quite a while now dedicated for just Kodi. On which it does an excellent job, as it is so much faster than other devices like Raspberry Pi for example.
Also I agree the “base” sound quality of the board is quite good on the standard outputs. However myself I am looking for more than quite good, to feed a SMSL DO200 DAC.
I saw just now that you replied long ago to my earlier post:
Apologies, but actually I was not active on this forum for a while as I “fixed” my problem by using the Raspberry until the new problem arose with the Hifiberry. You already properly closed the topic as you fixed the SPDIF in Volumio 3.
That is why I respond here now on your interesting comments.
Earlier I was not aware that I needed GPIO or soldering. When I bought the Odroid N2 I was so naive to think that the Odroid had SPDIF on the headphone output jack, like many other devices have.
Now back to your remarks. USB Audio is not bad at all, but it introduces noise. Now I can of course buy a noise silencer. But Audiofiles would say it is better to prevent noise in stead of filtering it out.
That is why I am now considering again to go through the SPDIF route. Now I saw your remarks about the GPIO. I have two questions on it now.
I saw this board:
Digitale Interface I2S vers SPDIF BNC WM8804 - Audiophonics
The reason why I like this more than the DIY optical examples: it has COAX which is less prone to jitter than Optical. I expect this is also a galvanic isolated board, but I could be wrong.
Furthermore as a NOOB I am thinking this may cover the otherwise needed DIY items like adding a 100 Ohm resistance and such. So I do not have to DIY all of it, like others seem to be so good at.
My second question: if I want to use the GPIO, what do I need for connectors, wires? Where do you buy such stuf?
I found this for example:
Insulated Female Blade Terminal 4.8mm 0.5-1.5mm² Red (x10) - Audiophonics
Is it the right one?
Also you said:
HiFi Shields are good, worth the money, but there are better alternatives.
Can you name some? I would be interested to apply one maybe.
Are there any other newer Odroid boards, with a HAT that puts out I2S, that you plan to port Volumio to? I would be willing to volunteer buying testing for you, if you need.
Sorry for the NOOB questions but I offer my eternal gratitude for your answers in return (for what it is worth 
).
By the way I know quite some about audio, but my Linux skills are very very basic. Not to say hardly existent. 
Another thing: I could not get into SSH: I tried all the default passwords, as “odroid” and “root” user. Could you PM it to me?
Thanks again!