Roller, I have to admit that you make a point.
I did some additional tests on different songs. The DAC-01A is obviously a lot more detailed (I could hear part of the song that I didn't know were there with the IDT). So I tried to pinpoint exactly what it was that I liked about the IDT. I think it is that the separation between the voice and the music is pretty clear (which might be because it does not play all the music and is much stronger in the mid range than in the lows and highs). On the other side, on the DAC, the music is kind of obstructing the voice (both with headphone and speakers (Wharfedale Pacific PI-10)).
This is somehow coming back to a problem that I have had for years: when I watch movies, if I use the IDH and headphones, I don't need subtitles, whereas if I sure DAC + Tube amp + Speakers, I often do. Moreover, one must know how I already heavily tweak the sound to amplify the voice (in AC3filter, I increase the centre channel and use the DRC).
Again, this is not related with sound quality. I bet that if I could listen to the movie on my phone, the voices would probably be pretty clear (phone are made for voice...).
I really which I could make my system better at playing these damn dialogues.... Any advice welcome.
Serge