When I got my HE-500, I listened quite loud, slowly lowering the volume over the weeks. Dunno why, but that will definitely have an influence on perceived sound, mostly bass and treble being more prominent. Pads wearing in as well can affect the sound. Else, there is the brain burn-in as well. I suppose you'd perceive the sound as more balanced after a period of brain burn-in.
Have a story to share with ya, a little OT. I have had some problems with my Stax pads slowly sliding up the frame (they are glued onto the driver frame).
So, I removed the glue, and got some double sited adhessive tape, and taped the pads back on (what a hassle!).
The point is, that the double sided tape is somewhere between 1 and 2mm thick, which result in more space for my ears, which I really like, and this was ofc expected. BUT (!) it has also impacted the sound quality, IMO negatively, resulting in a drier bass, and sligtly brighter sound which I tend to find slightly annoying. In the beginning I thought it was because I came from using the HE-500's exclusively over for couple of days, but the effect hasn't subsided.