Thanks. That Amazon link is the one I was looking at (albeit I'm in the UK). I like the inclusion of the dust plugs too.If you get the third-party clear case from Amazon here, you can pick it up how you would normally without fear of pressing the buttons. It WILL start yellowing (all of them do though) after about a year or so.
Sounds like you got some good listening going on already with it. Fantastic!
If you are looking for some amazing, closed back AE phones, I have found the Focal Elegia to be one of the best "bang for the buck" items you can currently get IMO. They were $1k originally, then everyone thought they were discontinued, but they are still being made, and you can get them for $399 brand new (though they DO come with a 3.5 SE cable which you can replace easily from standard 3.5mm jacks). Granted, they are NOT for everyone's tastes, but for me they come the closest (and even rival some) in sounding like an open back set that I have heard thus far.
I have both the HE400S (discontinued), and OG Deva (also discontinued for the R2R Deva now) and find them to sound amazing with the DX300 Amp11.2. Though I think the Deva sound even better on the Amp13 tube amp card.
If you aren't set on IEMs, I might suggest the iBasso IT-07. They pair stupendously with the DX300. I have all three amp cards, and it is my opinion that they pair the best with the Amp11.2 card (that you currently have), though it is pretty close with the Amp12 card (just depends on what you are looking for if you think one is better than the other). Amp11.2 has a bit more quantity of bass, and more upfront and clearer mids, where Amp13 is wider, better textured bass, and a bit more balanced in the mids (this all being with the IT-07 BTW).
Focals are on my radar but I want open backs, so the Elegia didn't make the list I'm afraid.
Good thing is I can't really go buying just yet, so plenty of time to read reviews and maybe audition some different models if I can find some real life shops to have a play in.