Driving the K712 Pro with the 9038D at 100% will result in:
2.8Vrms (voltage limit of 9038D)
41mA RMS (~90mA RMS is the 9038D's current limit)
ergo, current is not the problem here.
Edit: the Amp's Zout doesn't affect the AKG's frequency response either, so that's also not a factor.
The Y-splitter needs no special phone to function. It'll work with all type-C phones, tablets, and PCs.
It is configured so that you must have an external power supply plugged in. Otherwise it wouldn't work properly.
If you're looking for a short C-C cable to go along with a Y-splitter, then...
When connected to your PC, foobar will control its own software volume, the native Windows 10 volume slider will control the 2.1's UAC2 hardware volume, and HPToy will control the 2.1's DSP chip volume.
As you can see from the first measurement of this review, the Onyx outputs 2Vrms under no load (=plugged into an external Amp).
That is precisely line level for RCA DACs.
That being said, I'd be surprised if the A90 couldn't handle a fair bit more than 2Vrms on its RCA inputs.
The THD menu isn't all that difficult actually.
The first screen () allows you to set up three different THD settings, which will be applied depending on the 9038D's current hardware volume.
The three zones are nice if you have the right equipment and you want to optimize distortion across the...
The 9038D uses a regular old stereo TRS jack.
Inline controls or microphones are not supported because the 9038Q2M DAC chip inside the 9038D is a desktop-grade component designed purely for D->A performance.
It simply doesn't support creature comforts like that.
Lastly, please be aware that...