Interesting findings with crossfeed, I've finally come across songs in my playlists that show distinct differences with crossfeed enabled on the X9 as opposed to some subtle differences (if at all) and it's genuinely quite excellent. I thought maybe the Arya was naturally crossfeeding by default as commentary by others suggested this might be the case, seems not and I just had not come to songs in my playlist yet during my sessions A/B testing with it until now that are produced/mastered in a specific way that makes crossfeeding a positive thing for them. Songs that have no audible difference remain unchanged, so effectively can leave it on at all times I guess. All amps/DACs need a feature like this.
In this song:
Because it has been produced with strict left/right imaging for the background instruments, the bass strings and cymbals coexist in the same space in the stereo image in my ears on the left channel, can tell them apart obviously but they sound like they are coming from the same position around 7 o'clock from my head tracking.
With crossfeed enabled and left on the default 700Hz/4.5dB option the bass strings are lifted from that very left position and brought closer and it feels like the bass exists at around 8 o'clock position whilst the cymbals remain at 7. I disconnected the headphones and enabled it on the speakers output and yes there is some improvement here too but not to the same degree which was expected anyway.
Seal's Kiss From a Rose also has the same type of difference with it on and off.
Both Arya and HE1000se show the same here and both sound excellent.
Also observed after more listening between both headphones that where the HE1000se is brighter than the Arya, that brightness is not harsh or peaky like the Arya can be on the stock pads especially, the HE is just brighter without the harsh peaks that result in fatigue. On moist music I appreciate this more as it gives the impression or more detail, but on songs that are highly compressed or poorly mastered, this can result in a grating feeling in my head with the HE, whilst the Arya just makes those types of files sound mushy if that makes sense.
HE1000se is unforgiving to poorly mastered
The other two crossfeed modes shown above don't seem to have much noticeable benefit over not using crossfeed to my ears.
So yeah, crossfeed is great, but it is best experienced with songs that have been produced/mastered with specific type of stereo imaging, can just leave it on and experience the benefits when a song works with it, otherwise carry on as normal.