Ah OK both, It will be kind of pointless and sub-par audio quality wise...
In that case you only need a Analog RCA to Digital SPDIF converter, connect your Astro MixAmp as usual.
On the Astro MixAmp headphone/mic connector, connect Y Adapter (Headphone - Speaker splitter).
Connect the Speaker end of the Y Adapter to a 3.5mm to the Analog to SPDIF converter, then connect your headphones to the Arc Headphone input.
Now, this is why is pointless and sub-par (sound performance wise). First you will be wasting Arc functionality and paying a lot money for amplification.
In that arrangement in essence you are only want to amplify for your MixAmp audio output. In that case, a headphone amplifier will better and cheap, something like the JDS Atom, Objective 2 or Liquid Spark, for only $100 usd will do the job.

Now, this is why is sub-par audio wise.
The audio signal that comes from your console shouldn't be distort or compress, is very important.
When comes out of the MixAmp it has some level of compression (just like the image resize example), also the Chat Mix Audio adds more "noise", and finally MixAmp makes a bit of amplification. So at the end you have this compress, distort and a bit amplified audio image and on top of that you make it louder with an external amplifier, giving the double amp dreaded effect.
Granted, earlier MixAmps had a ton of compression, 2017 models are way better but I don't know how good "transparent" is the new 2019 release.
If you want Surround Sound, EQ's, and be able to use high impedance audiophile grade headphones, then
Sound Blaster X G6 will be a better choice over the MixAmp with an external amp.
SBX G6 has better dynamic range and raw power than the SB X7, still not as good as the Arc.
Now if you already have MixAmp 2019, then your best route will be to find a good Audiophile grade headphones that runs with low impedance (60 Omhs or lower) and high dB, like the GSP 600/500, ATH-ADG1X, HD599, Hyper Alpha, Cloud II and use DareDevil aka Emperor Potara tune EQ presets for these specific headphones. MixAmp will be enough to power them and it will sound as it should be through Potara well tuned EQ's.
As you can see there is many options and there's not a perfect device. MixAmp Chat Mix and Surround are its best features, SBX is in the middle with above Sound delivery, better Surround than MixAmp, and the Arc or dedicated DAC and dedicated Amp will be the best sound performance, but no Surround on those and messing with cables (specifically on XB1 due its limitations).