Excellent comparison between the IE600 and IE900. If I were to choose one I wouldn’t hesitate to pick the IE900. The midrange improvement from the IE600 is not something that I’d really notice. On the other hand, the bass and soundstage especially make the IE900 much better. But I would also be perfectly happy with just the IE600 because it’s indeed fairly close.
To make matters worse, these recessed MMCX connectors seem to have connectivity issues. Sometimes when I move my head, the sound cuts out for a split second. From what I understand, this is an issue with the stock cable as aftermarket cables don’t seem to have this problem.
That happened with both my IE900 and my IE600. A bit unnerving initially but, for both, it was solved with a few (i.e. not even 10) connections and disconnections while ensuring that the connector is firmly pressed on re-connect.