I had a full day demo with the C1 a few weeks back. It was on my Stax 009s and KGSShv Carbon amp. It is a great DAC and has super detail and a wide soundstage with full bass and good vocals. I was pretty sold on it for an hour or so playing around 10 favourite tracks inc some Trance, Beyonce and Rammstein. But it started to slide the rest of my time with it trying tracks of less well mastered material such as Cranberries and Evanescence which are bright recordings and can test a digital front end. Then it became fatiguing and very digital sounding. It had a pre-ring or post ring I am convinced, I am not sure. But the treble on less well recoded material had a viscous ring and edge to it.
The Stax 009 headphones fed by the Carbon are very transparent and fast but have NO distortions. But if there is a treble coldness or harshness it becomes fairly obvious. To be honest I was very disappointed. I really loved the build of the C1 and the architecture and engineering gone into it. It is an R-2R but oddly oversampled everything, and I presume has a filter? Regardless it was almost perfect on superb recordings but I couldn't live with it in the end as more than half my Redbook catalogue isn't that good, thus I was clock watching at the end of the demo.
Back to my existing Audio Note DAC 5 Special (with some upgrades) the exact same Stax system sounds, well, as good as the C1 but has none of the treble harshness I discovered. The DAC 5 also sounds fabulous through my Zingali Client Name EVO 1.2 I also use as well as the Stax 009s.
My thoughts were is the USB in the C1 a weak point? I was using that fed by my optimised Mac Mini (SSD, 8gig ram, 12V DC linear supply, fan removal) and Audirvana+ ver 2 asyncronious and with the TotalDAC USB filter/cable, the same as I use into my Audio Note.
It would be interesting to hear the C1 again but through the ethernet board option, as it did have a great soundstage width and probably a bit faster than my Audio Note. BTW I run my Audio Note through a passive so but like the C1 i.e. no pre-amplifier in the chain.
With high end digital there is no free lunch I guess, more detail and speed brings problems. Really well recorded material sounds superb, less well recorded material almost unlistenable. My Audio Note is slightly coloured, a warm sounding DAC I admit. I have tweaked the coupling caps in it and the tubes to bring it to 'my perfect ideal'. It has oodles of detail and pace but I don't seem to pay the price for that in listening fatigue. The DAC 5 is so old now, a design almost unchanged since 2008 for the V2 version I have.
Anyway, I haven't given up on the C1 and will get another demo with the Ethernet option. I don't want or have the budget for the D1, and I find that crazy to have to buy that at the cost. A DAC costing as much as the C1 should not require that iMO.
In the meantime I am going to try Rednet by Focusrite for an Ethernet music feed into my DAC 5. It is supposed to beat USB and can be had for 1K USD.