@dpac996 I do not think it is 2 DACs it is 2 signal paths that are combined. The analogy the company uses is digital photography were dark and bright sections of say a night photo is sometimes done by taking multiple shots optimized for both dark and bright areas. These photos are then combined to produce a clear image where the bright are not too bright and the dark is cleanly visible.
They plan on using this technique for various other studio instruments such as microphones and other things I have no clue about. The technique is called HDR-A and the D-1 uses something they call multi-path.
Last night I ended up working till 4AM and I was listening to my RAAL SR1a and the Schiit Mjolnir v3 amp until 2:30AM. This was after I had complained about the fatigue in my ear in the post above and then changed the top-level filter to F0 (default). No issues even so late at night and I was laughing at how good this was. I would have been more productive if I had turned the music off.
Yesterday, I also sold my $7000 MRSP RAAL VM-1a tube headphone amp. It was the very best way to listen to the RAAL SR1a earphones. My friend Danny sold me this amp 2x in the past and he was involved in the design. However, my used $800 Schiit Mjolnir v3 amp and the D-1 via XLR was as good and I dare I say maybe better than the VM-1a. Just crazy how good this DAC is and how much stuff I have sold because of it.
I have sold $14.1k worth of gear in the last 3 weeks and have huge improvements in sound in 3 systems. I have another $5k of gear to be sold. I look at this $12k cost of the D-1 as free money.
With regards to filters I will be staying with the F0 filter that is similar to many DACs that have these 5 filters. However, on every other DAC I could not hear these filters. On the D-1 I physically felt 2 of the filters. I only tested 2.
The other filter is the 2nd Order Harmonic Distortion filter. This filter is different from the F0-F5 ones. The HD filters are easy to hear. On my Yamaha NS5000 system I do not use a filter. On the Magnepan Mini I was using setting B at level 2, but I will eventually remove it as the hours increase on the DAC. On my RAAL SR1a, a bright phone, I use level 4.
I need to talk to the designer about these filters to understand the sonic signature of each. There are 50 permutations, and I have no time to experiment with all of them. I will meet the designer in person in 2-weeks at the Denver headphone meetup.
Does it come across in my posts how much I LOVE LOVE this DAC. it is easily the best piece of audio gear I have ever had in my system. Not the most expensive but the best by several orders of magnitude.

