One of the reasons we like the AKM4399 (and 4396) is that they use switched-capacitor filters to provide out of band noise performance that is more similar to a multibit DAC. As far as we know, they're unique in this regard. They're still delta-sigma, but they're a rather nice implementation of delta-sigma.
It's also a little ironic. One of the senior projects I did way back when I was getting my EE degree was a switched-capacitor, frequency-sensing noise gate for analog noise reduction. This was before the days that FFT and DSP became inexpensive enough to use broadly. It's neat to see that switched capacitor filters still have practical applications. Oh, the days of Z-domain analysis!
I'd wager that the reason the NFB2 sounds dark is that Kingwa is not fond of the upper frequency tizziness that plagues sigma-delta dacs (especially as the pcm1704 and ESS chips were problematic for him supply/design wise). I'm very curious if the Bifrost AKM chip (4399?) manages to avoid this sound signature. I have a Dacport LX with the AKM 4396 multibit and it sounds very similar to pcm1704/AD1865 = natural and no grating top end.