I own the EX-600 and IE8, and I would not say the IE8 is a definite upgrade. The Senn are warmer, and have a much thicker lower midrange compared to the Sony.
There is probably more bass quantity on the IE8, but as far as quality goes, I think I would give the nod to the Sony. However, it really depends what you're after. The Sennheiser is slightly softer, but also more organic and has more weight to it.
Regarding soundstage, I would not say that one is better than the other. There is however more air in the Sony's presentation.