I think you misunderstand what i'm referring to.
You don't use headphone amp to supply to speaker. If you want to use it as a dac, you just have to choose DAC and use RCA line out at the back and connect to speaker amp or active speaker, which means bypassing the headphone amp.
Also a lot of the DAC comes with line out, especially desktop unit.
Fostex HP-A3 is exactly having same function as X1s except X1s has build in preamp for speaker. If you do not need this function, just choose pure line out and it works exactly the same as Fostex HP-A3 and Audinst HUD-MX2.
To make it simply, I will write down few combination for you to refer to:
Speaker system
DAC/ Sound card > analogue line out (3.5mm / RCA / jack / XLR) > Integrated Speaker Amp > Speaker out (banana plug or Y plug > Passive Speaker
DAC/ Sound card > analogue line out (3.5mm / RCA / jack / XLR) > Active Speaker (with build in integrated speaker amp)
The terminology might sound confusing. Usually we call it headphone amp but some also call it headphone preamp. Headphone preamp is different from speaker preamp.

Check out the in /output.
Audio in meaning, bypassing the DAC and just use the headphone amp, where X1s receiving analogue signal from another DAC.
Audio out is the analogue line out that I'm referring to. Bypassing the headphone amp and just use it as a DAC.
But one more extra step which you have to choose at the front of the unit, whether to use speaker pre-amp or pure DAC
Coaxial, optical and usb wise are for digital signal.
** To add on, if you don't really need a volume control at X1s or any other DAC for your speaker, then actually don't have to bother using the speaker preamp function.
Some need it is because their speaker system doesn't come with it.
I hope this clarifies.

lol, I understand what you mean, I've try use DAC line out straight to power amp, but then no volume knob to control volume, unless hook up to an integrated amp, but then I'm using power amp, also for headphone I'm using O2 combo. If I remember correctly Aune X1S does not have preamp volume control for the DAC right? That's the feature a lot people love to have, marry me that button!