Thank you for your compliment. To be honest I have only ever changed the volume via my Amp. I haven’t used the Dac for a few months as I mostly just push everything through my Dap and obtain extra amping through my Atom if needed. I am sorry I aren’t able to help on this but maybe somebody with more technical experience with Dac’s as a whole like @Torq could help you out.
I tend to listen to my iems a lot now and use my Dap. I agree with you though that the D50 is an impressive little Dac for the money. I understand that there’s a new model D50s coming out soon, it may even be out already. Again sorry I aren’t able to help you out.