I am still in progress of writing review. For now my 2 cents - I have just tried to listen S6RUI from Astell Kern SR15 and their synergy is like several classes above what I have heard from S6 before. I have tried S6 out of my Phone and Hiby R6 and they felt like neutral to bright iem with overall thin sound while on SR15 S6 sound full, balanced, without any peaks at all and very emotional. Incredible synergy - everything is in place. S6RUI ------> times better than S6RUI on Hiby R6 or my phone, I have not heard such synergy on Ibasso IT04 or Kinera Odin and SR15, they are like made for each other.