My testing for this comparison was using my REFERENCE Mojo2 system. Mojo2, like all Rob Watt's DAC design, has an internal PCM signal path. Note that Mojo2 uses a 104-bit DSP design vs the 64-bit DSP in other chord DACs. This will provide it an advantage in digital signal processing.
With regard to DSD. DSD input is converted to PCM and this, according to Mr. Watts, results in distortion. However I thought that DSD sounded great. So any distortion was i) imperceptible to my ears through my headphones, ii) beyond the resolution of the Mojo2 analog section or, iii) hidden by PGGB's excellent DSD modulation and Mojo2's DSP design. The theoretical PGGB noise floor (measured by remastero RASA) is much worse for DSD than PCM ...so consider subjective bias to be dominant when assessing DSD quality.
With regard to 24-bit vs 32-bit PCM. In both cases, PGGB noise shaping is absolute superb so distinction between the two is based on how much acoustic energy difference is audible. IMO, with a USB interface, the difference is imperceptible: with several A/B tests, I could not hear the difference. However, my users have indicated that SRC-DX is their interface of choice ...which, due to avoidance of a USB interface, generates less RF noise inside the DAC and this sounds much more clean and transparent. SRC-DX supports the SPDIF maximum bit depth of 24-bits but the benefit of less RF noise is unmistakable: after listening to SRC-DX, going back to listening to USB is torture. Believe it.
The other aspects of my system that helped to extract the best sound for testing are the optical link from source (OPTO-USB), the best DC power possible for Mojo2 (PURE-DC) and the extreme of using radiated RF isolation (RF-STOP Faraday Box). I had great enjoyment in listening to my reference tracks.
So - the takeaway from my comparison is:
- DSD playback on Mojo2 is subject to theoretical distortion that may or may not be audible. If you want the objective best sound with Mojo2, stick to PCM. With DAVE, it may be possible to use the DSD playback mode for benefit of digital processing and an improved analog section.
- USB playback on Mojo2 reveals equally great sound with either 24-bit or 32-bit PCM. If you insist on USB, you can pick either for best sound.
- DBNC playback on Mojo2 using SRC-DX (at 24-bits) is definitely the winner. Versus USB, DBNC has equal clarity and detail - but (mysteriously) the slight reduction in internal RF noise (no USB clocking or processing) gives DBNC the edge in reality creation.