I personally don't think it'll be necessary. They're honestly not hard to drive. But then again I tend to have fairly sensitive hearing and listen at lower levels than most of the people I know. As far as levels go, it seems fine with my Zune, and even respectable with my lame ass Android phone. You shouldn't have too much trouble with an iPod Touch.
BTW, even if I did think an amp were important for these cans, I probably wouldn't go with an E6. I had an E5 ages ago and found it spectacularly unimpressive. For me, it muted highs to such an extent that I remember thinking that it was like hitting the Dolby B NR switch on tape decks ages ago. And from various E6 reviews I've read, it doesn't seem that different from an E5.