Wait, could it be that your DAC starts some sort of action internally for temperature control or similar. Something that under "normal" circumstances wouldn´t really be noticed. But since you have suppressed the noise floor more than what they have even tested it in, probably 🙂 , the sudden raised noise floor by this (whatever it is doing) is actually noticeable in your application.  Just thinking out loud...

Or from your PC streamer, which maybe makes more sense?

This still wouldn´t make the sound dark, in my mind, but i have been surprised before... 🙂

Good morning Tobias, Yes that is what I think as well, the PC. I remember that for a test I also chose my wired tablet as an output. It was dark as well, but also the television when using Netflix while connected with Toslink to the DAC. Also when using AES/EBU from my Pink Faun streamer to the DAC. The motherboard of the Pink Faun streamer was replaced 4 times, due to this issue. Also I2S is used and USB output. Those were all high grade audio modules.

I also have a NAD M12 dac of which I replaced the stock VCXO clock with an ultra OCXO. The OCXO was fed by a Farad powersupply. When the NAD became dark back in 2019 I could switch off the clock and the sound didn't change. So the NAD seemed to play on its TTL, but when I reconnected the clock after 45 minutes you could hear everything cleared up for a while.

Also last week I built a completely new PC, Asus Prime, intel I5 with a Seasonic Fanless power supply which is a SPS, but a pretty good one. This PC is compared with my other PC which was built 7 months ago. It has a Seasonic as well, MSI motherboard and Intel I9 one of the very fast. They all suffer the same problem. In the near Future there will be a Farad ATX which shall replace a Seasonic. This Farad ATX will outperform the Pink Faun powersupply. Both are from the same hands.

But also when the DAC/ Streamer is connected directly to the Router. It will become dark. Even when a CD player is hooked up to the system, it will remain dark.
There’s one thing they all had in common and that is the ethernet was connected to the system. Actually when I replace the Cat5e SFTP with CAT8 the problem will become worse. Also when I put a few powernet filters in the chain, the problem will become worse.
Yesterday it seemed that even the AQvox switch was too much. The system went dark later in the evening. It cleared up after I had put the normal D’Link DGS108 back in the chain. Actually the AQvox and the DGS108 are the same switches, but the AQvox has higher noise reduction

Hmm, are you saying that it even gets dark if ONLY the CD player is physically connected to the DAC?