What to choose in Audio filters: Apple core audio, isotope or soX if I’m not using any upsampling and just need the music to be transferred without any changes? Using USB connection from Mac mini to my Hegel HD30 dac, some times also using UPnP connection directly to my dac
If in filters settings you are at none,
It doesn’t matter 
. Mine is at izotope with my parameters but for now i don’t upsample anymore. So i just put it at none.
But why then I’m hearing some difference in music when using Apple core audio, isotope or soX even without upscaling? )
For example violin in Paganini 24 caprices op 1 is more transparent and has mor “nerve” when using Apple core audio
if you are not upsampling, A+ is using Apple CoreAudio…
Then, if you have an older system, not Sierra Or High Sierra you can use instead the Direct Mode Damien made that bypass Apple CoreAudio, better for some. I’m not an expert by far on that so… maybe someone can correct me if i’m wrong 
If you choose to Upsample you have 3 choices for the converter.