i think it's just some really subtle ambient effect for the background even if it's super super low volume. in the high end maybe? fucking love it though! really creative mix, you could probably notch the piano down a tad to mix in a bit...just blending the piano a bit to the mix, the tones really neat!
yeah i don't think you need any more sub harmonies or shit like that just something like a wind chime or maybe a pad...i leave that to your discretion. awesome jam!