Personally, I don't really agree with
@snellemin that the Aune X1S is underpowered for the 400i. I could get why somebody would not like the pairing with the 400i (the X1S tends to sound a bit bright IMO) but I'm not sure why anyone would say that the X1S can't drive the 400i well.
It has more than enough volume (I listen at relatively low volumes so I only had the volume knob at 10-11 o'clock, but even for someone who listens loud I can't imagine them going beyond 2-3 o'clock). And the sound had decent enough quality to it (very punchy, good soundstage, and though its not the most refined amp out there the 400i didn't sound excessively bright or grainy like when it is underpowered).
You can look at my review for the X1S for more details linked here, but unfortunately I don't go too much into detail into this specific 400i/X1S pairing: http://www.head-fi.org/products/aune-x1s-32bit-384khz-dsd-dac-headphone-amplifier/reviews/14366