I think that only
@2ko is the other one who has the same "complaint" as me. He thinks that he's not loud enough either.
Thank you for sharing, it gives me a better perspective.
So, at the moment, I'm using it around -10 or -5dB. If I play with the equalizer a bit, like decreasing the pre-amp (by about 6-8 dB) and increasing the lower frequencies, it won't get loud enough even at -0dB.
I also find that going into high gain changes the sound signature. The smoothness that you find in the low gain is not present anymore. So, I will avoid high gain where possible.
Even my LG V30 manages similar output compared to the DX3 Pro. So, I think that I might return it and go for a standalone DAC and AMP.