The J3's HP out is just as good as a line-out, so a 3.5mm-to-3.5mm cable will do and the E11 includes one.
The J3's line-out only works with videos anyway. For your iTouch, get the Fiio L3(?).
Your J3's battery life will improve mildly when hooked up to the E11, or any standalone amp for that matter.