I have exactly the same setup which I have already upgraded a bit, so I feel I can comment this issue a bit.
One thing that your setup is missing tho is a Line-out-Dock aka LOD. When you feed your amp with a signal that bypasses the iPod amp the sound really improves. Then the signal is amplified only once which is naturally a good thing. I personally opted for Qables LOD 'cause it's an europe based company and the shipping costs wheren't too bad. So investing 40€ in a LOD would be a good idea.
What comes to my setup, I was pretty happy with all those parts, but upgraditis just bit me. First part was the Corda Headfive, that I just HAD to get since they were slowly running out. A very good decision on my part
MS-1 and Headfive really shine together. For this reason I could recommend the new Arietta which is supposed to sound pretty much the same as H5.
My other upgrade was moving up from those bass-heavy CX300's. I liked them, don't take me wrong, but I just felt that they were the weak link in my chain. So I bought me some Westone UM1 IEMs that I'm also very happy with. The amount of bass got reduced a bit, but that just made the sound more balanced. Otherwise the sound is much more accurate and the isolation is from another planet.
So that's what I ended up doing, coming from the very same starting point as you. Hope this gives you an idea or two or atleast educates you a bit of your options