^That was me, and it's not a side by side, as I no longer have the borrowed pair, but at the time that was my impression. The way the detail is sharper less syrupy compared to the W4 or SM3 or.....heck most of the mid centric stuff I've heard.
The thing I noticed is the bass response difference (of course), between the two. I even told the owner in conversation, that the bass response in most IEM's tends to "get in the way" of the details. To me that's the largest difference in focus on the higher tier IEM's, detail or balance.
But you're right it all comes down to the listener's ears too. Etymotic is not "balanced" to my ears, they don't sound live. They are detalied, but not life-like. The 4.ai is warmer, but still along the same path as an Etymotic er4 (to my ears), although the soundstage and bass extension are also better. Then the SM64 comes along with the treble and mid detail, and yes perhaps not the treble harshness you hear, but I attribute that more to the bass 'distraction' if you will, than the treble response. Just my theory anyhow. But the Heirs were not harsh to me. I get more strange peaks (and sibilance) out of the Westone 4.
The note presentation, depth of field, transparency (if I understand that term correctly), crispness and general upper mid lower treble texture all reminded me of the 4.ai.
Maybe my ears are crap. I wouldn't be surprised, I am getting older....