Output Impedance (HP) BAL
Even if those relatively small numbers are correct, with 18 Ohm Headphones, every increase in output impedance matters quite a bit.
I'm also no expert on these topics, but if I'm correct you're losing around 1/18th of your available power into that 1 Ohm resistance when using SE out. On BAL out you then lose around 2/18 (= 1/9) of your available power, so the efficiency drops.
"Losing" power here does not mean you actually lose it, it means it gets used inside the output's resistance and transforms into heat, not being used to drive your headphones.
At 300 Ohm the Amp's output impedance does not matter too much, as most of your power gets indeed "used" in the headphones resistance.
Also, as others already pointed out, you need a lot of current for 18 Ohm low efficiency headphones. The Zen Can seems to struggle with that current delivery, as you can see in Amirs Review.
I think you also mentioned the Schiit Asgard 3 as a potential alternative. While it puts out a lot of power into low impedances without clipping, the distortion rises quite a bit, even at 33 Ohm. The distortion performance will therefore be worse at 18 Ohm (it will also clip earlier, however Amir did not test impedances lower than 33 Ohm).
So yeah, if you don't mind distortion, you should get a lot of power from the Asgard 3.
Another potential alternative you mentioned is the Topping L50, which does not really struggle with current delivery whatsoever (see Amir's review).
I for myself don't really get the "its too analytical", it's just playing the music you feed it, without adding distortion products.
Either way, you really don't need that much power for the Edition XS, you just need a lot of current, so I would search for Amps that can provide that without clipping.
So, getting back to our actual Topic here, the wonderful Edition XS
When it comes to low impedance and low efficiency headphones like the Edition XS, one of the most important graphs in Amirs Amplifier reviews is power delivery /clipping behavior for different impedances. Thats where you can get a sense of the amplifiers capabilities for these kind of headphones.