Warning: this post deals with user experience and not measurements. Reader beware.
@antcollinet the comparison is two channel digital source into my AVR from a disk drive via HDMI or toslink versus digital source into an external DAC (toslink), and analog out of the DAC into my AVR in Direct Stereo which turns off all digital circuits in my AVR and it functions as essentially an integrated amplifier. The AVR I am using is an Arcam AVR400, which is about 12 years old now. The external DACs I have used with it are the Audioquest Dragonfly v1.0, Arcam irDAC (v1), Chord QuteHD and Chord Qutest.
To my ears with extensive listening in two channel only (no processing tricks other than room correction for the internal DAC, not the external) at multiple volume levels and using hundreds of digital recordings at 16/44.1 resolution, there are very clear differences between the internal DAC of the AVR and the external DACs. When using analog in from the external DAC, the subwoofer operates with high level speaker terminal in - and room correction has been dialed in for this setting as best as possible using a sound meter and listening. Any sound leveling was done using a sound meter - not voltages, but these differences described here were consistent over a range of volumes and repeated listening sessions. I have blind tested other listeners with my system, and these perceived differences are consistent and not subtle.
My room is purpose designed with a floated ceiling and one side wall completely covered with books and record albums and a concrete floor with a large throw rug. It is a little dead with respect to high frequencies below 60 db, but above 65db average volume, very little coloration compared to other listening environments I have experienced.
The DAC in the Arcam in two channel sounds “brighter” with high frequencies emphasized to the point of sounding “brittle” and fatiguing at higher volumes. The bass is very clear and tight with no bloat in this setting, quite good. The soundstage is wide and a bit hollowed out in the center. I have run the room correction multiple times and while the AVR sounds terrific to my ears on multiple channel Bluray material, two channel redbook recordings never measured up.
In comparison, the first version of the AQ Dragonfly with a computer as source was not an improvement. The Arcam irDAC was different and better than the internal DAC in key areas. Tone, how “real” voices and instruments sound in the room were improved to my ears. Listening fatigue was reduced, especially at higher volume. The soundstage was collapsed in the center and did not extend at all beyond the front speakers compared to the internal DAC of the AVR. The bass was less precise, and this was the trade off of operating without room correction.
When I upgraded from the irDAC to the Chord QuteHD, the detail retrieval increased, the bass tightened up some (still not as good as using the internal DAC with room correction), and the soundstage opened up considerably. Unlike the internal DAC with room correction which pushed the sound to the left and right extremes, the QuteHD precisely positioned instruments and sounds across the soundstage. Impressive feat.
None of these external DACs described so far have been measured by ASR, nor has the AVR400, although the performance of some newer Arcam AVRs have been reviewed by Amir, and the weaknesses of their digital sections in two channel and the generally good performance of their amplifier sections have been reported on this site.
Switching to the Chord Qutest (which has been measured by Amir) there was a slight improvement in all aspects of reproduction over the Chord QuteHD. I noticed slightly more detail retrieval (on headphones at least), more precise articulation of soundstage and cleaner bass. The improvement to my ears between the Qutest and the QuteHD was smaller than the difference between the QuteHD and the irDAC. The Qutest measured well in Amir’s test and has the best overall reproduction of redbook quality recordings of any DAC I have used to date.
I have not yet tried in my system one of the inexpensive Chinese DACs that measure extraordinarily well and compared that to the Chord Qutest. I might try one or several out of curiosity. My conclusion from all of this, based on Amir’s testing and my personal experience I would not assume you are getting highest fidelity reproduction in two channels from digital sources using an AVR with the internal DACs. But YMMV.
kn