Now that I have sent my AK240 for mod with Vinnie, I am back on the 901 balanced card. Well let me tell you something: with the NT6 the 901 is just incredible, much more musical and analog sounding, effortless and authoritative than the AK240 with the NT6. The AK has more speed and clarity but loses by a fair margin on this combo. Difference might be less strong on darker iems...the NT6 is brightish and pairs wonderfully with the 901...I am really hoping the mod will bring the AK closer.
Personally I really disliked the NT6 Pro / HM901 pairing.