Xonar STX II seem to be a much more expensive card but:
https://www.audiosciencereview.com/...surements-of-asus-stx-ii-pci-sound-card.4915/
2.1+ Vrms. 0.001-0.0015% THD+N. But around 97 dB SINAD. Then again the later is still just above CD quality right? Higher than JDS Element DAC for instance. Lower than Topping D10.
The Gigabyte B8 motherboard he claim has 107 dB SINAD.
"The J-test signal toggles all the sample bits to the tune of 250 Hz. This should be invisible since it is being applied to last bit of 24 bit words. Yet, we easily see its spectrum here indicating bleeding of digital circuits into analog."
Higher noise floor here and there against the motherboard, and higher just about everywhere than the Topping D50 (but how fair is that?)
"You lose accuracy in as low as 11 bits."
"I did not test the headphone output. It has a 10 ohm output impedance which is a bit problematic."
As for if any of that applies to the Xonar DS I don't know but I think that was a much cheaper card. However maybe it's better than many onboard solutions. I don't know if the STX II really is better than it or not. Or whatever the stuff above is correct.
With low impedance headphones my computer with onboard ALC1220 / SupremeFX made a lot of noises even with idle load and more with game however I think using a grounded power cord solved that. I don't know the conditions for the test above.
https://www.audiosciencereview.com/...o-measurement-of-lg-g7-thinq-smartphone.4468/
I don't know if LG G7 and V30 is exactly the same or not but:
1.04 Vrms, 0.00027-0.0004% THD+N, 107-111 dB SINAD.
"Output impedance was a low 1.6 ohm"
"The LG G7 ThinQ DAC produces exemplary performance."
"As we see, the headphone amp does not like low impedances. It sharply drops output voltage resulting in very low power outputs. This is the opposite of external headphone amplifiers where lowering the load impedance increases their output power."
3.6 mW @ clipping for 33 ohm, 2.5 mW for 50. With 100 ohm it did better with 50 mW.
So the LG G7 seem to have better quality audio than the Xonar STX II. But weak power.
https://headfonics.com/2018/07/xduoo-x3ii-review/
"The X3II dishes out 210mW, roughly, and that just isn’t much at all for anyone using a Planar or anything above 120Ω."
Maybe phone with just amp is fine?
JDS Labs Atom amp has favourable reviews but may not be great for portable usage:
https://www.audiosciencereview.com/...ents-of-new-jds-labs-atom-headphone-amp.5262/
at 33 ohm: "You get one full watt of power at just 0.0009% distortion. Incredible."