I've done some testing, too, and I can never hear the difference using a pair of Fidelio X2s through a Dragonfly 1.2. Good, saves me some space and worrying about having the highest possible quality.
Also: I'm toying with the idea of returning my X2s. They sound fantastic but the highs are just too fatiguing for me.
I've listened to the sennheiser momentum over-ears a lot and they're the closest I've come to my ideal sound signature except the fact that they're closed and only have an okay soundstage (and get a bit uncomfortable after an hour). They just have this nice, slightly warm and thick sound that I find really pleasant and satisfying while also sounding fun and musical. Yet they never ever get harsh. If I could find an open, more comfortable version of that I'd be golden. So I'm looking for something with
- an open, airy sound with a good soundstage
- laid-back, non-fatiguing highs
- decent bass
- comfort (preferably over-ear)
- max $300
I tried the HD598s but they were too bright for my taste. Going to give the supposedly darker HD558 a try next. The HD650 is out of my range money-wise, and the HE300 is really hard to find here in Northern Europe.
Another option might be to keep the X2 and use an equalizer to take the edge of the treble. I mostly listen through spotify which doesn't have an equalizer, so I tried this app Boom on my mac, but it made everything sound strange and digital. I guess I could buy a FiiO E17 which has an equalizer, but seeing as I have the dragonfly it feels a bit unnecessary to have two dac/amps. Would love to hear some suggestions. I love almost everything about my X2s and it's frustrating as hell that the treble has to a dealbraker.