I owned ES12's first and liked them a lot, so much in fact that they had me wondering how good the original Robin Marshal ES11's were. I managed to pick up a pair that were cosmetically near perfect (on Google images they are the first you'll see) so I was able to make a direct comparison between them.
The two speakers have an almost identical tonal balance, legendary midrange as you'd expect, but the ES12 having a slightly fuller and softer bass. Compared to the ES11's though they do not resolve as much inner detail, do not image as precisely, nor do they sound as rhythmically fast. Sample variation perhaps? Well no as I bought a second pair of ES12's in order to get some dedicated open frame stands and again I felt they were actually a step backwards from the ES11, I heard the exact same deficiencies in comparison to the ES11. I cannot emphasize enough the importance of the Epos stands, they really tighten up the sound and make them a much more focussed sounding speaker, together they work as a system.
ES12 on top of Epos ES11. ES11 sitting on their dedicated stands.