I actually just picked-up a pair of IE600's a few days ago, before seeing this post. There was a $400 off sale on Amazon and I was eying them - been wanting to take a step from the Shure 425's I've been using for close to 15 years. Figured if there is an issue, it's easy to return to Amazon.
Anyway, I'm really loving these IE600's! A nice step up from the Shures I had. The bass is nice and rich, love it. Just a few days in, but so far so good. Playing around with the eartips. I've partial to foam tips with the Shures and I think it will be the same here. I just get better isolation with them. The larges are best but a little tiring after 15 minutes. Will probably go with the mediums in the long run.
I'm playing these through a Helm on my iPhone, with files coming mainly from Tidal.