I've been listening to the ZX300 for 1 week now, and one thing is sure: I love it!
The issue is now to find IEMs that are suitable to my tastes - I have a tendency to favor solid bass and mids, with slightly recess trebble.
So far, none of my headphones has been a proper fit for the ZX300. The HD650 and the ZMF Atticus need more power than the ZX300 can provide, although the Atticus didn't sound bad at all (I was using them on high gain, of course, but on the single ended output). The Oppo PM-3 isn't too bad, but needs as much power as the Senn and ZMF, and is uncomfortable due to the cups being too shallow. The V-Moda M-100 is pretty good, but has a tendency to get muddy when there's too much going on.
Of all my IEMs that are still in working order, the Sennheiser IE8 is the most decent, but still not satisfying. I miss my Shure SE530, whose left channel died over 10 years ago. If I remember well, it was really good, and the IE8 that replaced it was never up to its level.
So far (and it can change, of course) I'm still researching possible IEMs in the $500 to $600 range, and possibly distributed in the US. The AKG N40 is still on the list, but I've added a couple more, and would like to know if anybody in this thread can give me some insight about them: the Shure SE535, which in theory I should like, since I liked its predecessor, and the Empire Bravado, which can be sold with a 4.4 balanced cable.
Thanks!