Hi
Walle90.
Sorry I'm behind with my inbox msgs, saw you write me there.
Not sure what do you want to improve.?

The reasons of sticking with Astro MixAmp is to have Chat/Game audio Mix control, sacrificing better sound quality and resolution, and use VRR and 120Hz on 4K. MixAmp + external amp just doesn't sound as clean, precise and fast as Mayflower ARC DAC.
If it's Chat/Game audio mix what you need, then you can make it work with Mayflower ARC MK2 .
Set on your XBox that Chat Audio goes only on your Headset.
Then connect a Y Splitter Headphone out / Mic In adapter, on your controller.
Connect your modmic to the Mic In.
Connect a 3.5mm male to male extension from the Y splitter Headphone out to the back of Mayflower Arc MK Aux In.
Keep using HDMI Audio Extractor to connect TOSlink Optical to the Mayflower ARC MK2 TOSlink Optical In. Mayflower ARC set on Optical Input.
This way, Mayflower ARC decodes pure digital Game Console audio coming from HDMI.
Modmic goes thorough the controller
Chat Audio that comes from the controller to the back of ARC gets digital mixed (unique feature) into your headphone, you can use volume control on Xbox to manage Chat Party volume. Less messy than having 3 devices and many more cables.
Gaming Performance wise, ARC sounds better.
Now if you think you need to improve something in your chain... headphones man, those always make a bigger impact.
Beyerdynamic DT990 or Tygr 300 are worth to give it a shoot. Compare to HD660S, Beyer dynamics on competitive shooters its wider soundstage and very revealing imaging gives you an edge, however are bright and crisp, can tire your ears sooner than HD660S which are more relaxed and easier to hear on long periods of time.
iFi Zen CAN + External DAC + EQ like loki are great as entry level of HiRes music listening.
Using Zen CAN with XB Headphone Output is great option to use high impedance headphones, but doesn't beat Mayflowers, where the DAC makes a more precise sound picture, where you can distinguish every sound cue its position and texture, Zen CAN taking the sound from the controller DAC which is lower res... can't do much. But this setup is recommended if you want to keep VRR and 4K @120Hz.