It's surely possible that "Supreme Fx" hyping of Asus is just marketing BS and actual implementation is worser than what Realtek codec alone in reference design would do.
Because DT770 certainly shouldn't be flat with its strong rumbling bass of closed design and likely some "Beyerdynamic treble".
What's the use (gaming/movies/music?) and do you have for example also speakers in PC?
Get a FiiO E10K from the jungle site for about £60. Try it and if it does a better job than your onboard audio, it'll be a decent cost effective choice. If you notice no difference compared to your onboard audio, then send it back.
If there is no difference, then maybe time to look at different headphones.
Hi
Just had a look at the FiiO E10K and it says its Suitable for headphones impedance between 16 - 150Ω Will that work for 250 Ohm
As above; it can drive headphones with higher impedance than the rated 150 Ohms. FiiO are quite conservative when it comes to power output of their devices.
I've got a FiiO X1, which specifies headphones of 100 Ohm, but it does a pretty decent job of powering my 300 Ohm HD600's.
Or more accurately impedance is rather bad measure for how demanding headphones are.As above; it can drive headphones with higher impedance than the rated 150 Ohms. FiiO are quite conservative when it comes to power output of their devices.
Some of earlier Hifimans were more like small speakers in their power requirement, while having "easy" around 50 ohm level impedance.