In my short time with the IE 600, I found that it has more similarities with the IE 900 than differences. Tonally, the two sound quite similar, with shouldn't be surprising given that they are based on the same driver and are tuned by Sennheiser to appeal to discerning listeners, aka audiophiles.
The difference I found was that the IE 600 sounded more intimate, less wide, and less spacious. Perhaps that's due to the fact that its sound is piped through two resonator chambers instead of the IE 900's three. The mids also appear to sound more balanced than the IE 900's, while the treble seems to be oh-so-slightly brighter.
For the full review by HardwareZone, read here: Sennheiser's new IE 600 IEMs feature hand-finished 3D-printed zirconium shells (Updated)