At the risk of being "shot down" - I'll give you my 2 cents worth. I do own the 535 (Ltd Ed)
I've tried them (and also originally the SE425) with -
An E7, an E11, a porta-tube, and the Arrow.
IMO any improvement you will get (beyond psycho-acoustic) will be extremely minimal. Here's why ......
The 535 were designed as a highly sensitive, low impedance IEM to be used straight out of a portable player. Their sensitivity is 119 dB SPL/mW, and they have an impedance of 36 ohm. So any linear DAP with an output impedance of 4-5 ohms or less should have the correct damping factor, and enough to power these, and power them extremely well.
Assuming your iPhone is a 4, 4S or 5 - then you're covered.
On my 535s - I rarely get past 25-30% volume straight off the headphone out. The iPhones are low noise, generally very flat - and actually sound good for a portable device.
If you want a good parametric EQ - look at some of the apps - like Equaliser - then you can tweak to your heart's content. With the money you save - buy more music.
.
Above just from my own personal experience. For the past 6 months+ - I listen to my IEMs straight out of the iPhone, and couldn't be happier. No bulky portable solution. Great sound. No big cash outlay for little to no gain. If you were talking hard to drive IEMs or cans - then it's a different story.
These aren't though.
Only other reason for getting an amp (for the 535s) would be if you are getting hiss straight out of the player (impedance mis-match). If this is the case - then an amp with a very low output impedance is often the ideal solution. But in this case - with your iPhone - I very much doubt this would be happening. it doesn't on mine.
Disclaimer - I don't have golden ears (actually more likely to have 'tin ears'), I have tested amped vs non amped blind (and volume matched to the best of my ability - my wife did the switching) - and I noticed practically no improvement. YMMV.
