[Derivative]

Whether is worth paying the price for IE900, only you can decide.
For me, IE900 was clear better than IE600.
Better bass, better midrange - I found the (generally criticized) dip being ideal for metal, while the IE600 sounded too shouty here.
For the highs, again IE900 is the winner, more detailed, more refined and not being too energetic/fatigue, just at borderline sometimes.
But I would'nt have paid the price for IE900 if not discounted under 1000 usd.

I second this - have owned both and the IE900 is worth the extra money and is a far more interesting proposition than the 600.

I recently plugged mine into my Little Dot tube amp, worked really well as the amp brought a nice warmth and coherence to the IE900s polished technicalities. This versatility makes me like them even more.