I won't say I am disagreeing with Mike's opinion, but I am not in full agreement either. I can see my Mike might prefer E17's internal amp over E9, especially when paired with low impedance headphone. For those who know their E9, you should know there are output resistors on the headphone jack that will color the sound of low impedance headphone (10ohm on 1/4' jack and 33ohm on 1/8' jack, IIRC. These are there to stabilize the opamp output). I am listening to E9 via the 1/4' jack with IEM > 100ohm (ER4S, RE262) , so coloration (from output impedance) is minimum, if any at all.
Now lets talk about the difference first: E17's internal amp has wider soundstage and crispier sparkles. The sonic energy is mainly focus on the upper vocal and up. E9 on the other hand has (a lot) more power. It has excellent control over the bass note which is deep and well bodied while remains tight. It also resolves lower vocal better. The way I like to see them - E17's internal amp is like a female punk rocker: aggressive, lively, and demands attention. E9 (w/ E17) is more like a tenor: powerful, warm and full of authority. I can't really say one is (very obviously) technically better than the other. They are certain things they are better than the other, but not in everything. So my recommendation falls back to what they do best in the practical world - if you are using anything below 150ohm, E17 alone is enough. If you are using anything beyond 300ohm, E17+E9 makes better sense. For those in between, I think both can work out well (in different way) depends how you like to take your music.