I used to own the er1 es1 and the ea1 and now currently just have the new EMX1. It is great but it doesn't replace either the ea1 or er1. The sound has more of everything but i still miss the others. I would say that the earlier electribes have a lighter more liquid sound whereas the newer ones are more "professional" sounding. I think there is room for both really in the way a Juno 106 isn't replaced by a Minimoog.
I'm currently saving for the ESX1 that matches my EMX1 along with re buying the original ones. They complement each other very well.
However i wouldn't but the es1 again as the ESX1 does alot more and the same.
Just my 2c worth!
Lifer