You won't get dramatically better performance from the 598 out of an iphone vs out of an amp, but it can benefit from it as I said a few posts before. Even just a slight improvement can sometimes mean the difference between rockin' out on a track and simply letting the track finish. It's those magic moments when you go - "da*n! That was awesome!"
Can you have those moments with the 598 on just an iPhone? Absolutely. But others need just that tad bit more to push the music quality over the edge that allows you to have those 'moments'. When that happens, spending a bit more on an amp - even for a 598 where the improvement is not monumental - is worth it for some people me being one of them.
Besides, a FiiO E17 is not that expensive. You can find one for around a hundy in the for sale section. Well worth it for the price.