I started with the HD660S which are darker. When I first got the HD600 I found them fatiguing and started to wonder if I made a mistake. I decided to use them exclusively for a week starting with songs that aren’t as high(only male vocal songs) then let my brain adjust to them. After the adjustment period, they sounded absolutely awesome and have been my daily pair since. They just sound so right. The 660S have gotten much less use since then, they sound muffled by comparison.

Headphone: HD600(Sidegrade from HD660S) couldn’t stand how bright they were coming from the 660S but after my brain adjusted to them, they became my daily drivers, they just sound so right

IEM: Moondrop Kato

AMP: Feliks Echo Mk2

Everything Started this year so in order of purchase: Moondrop Kato -> HD660S -> Moondrop SSR -> HD600 -> Beyer T1 V2