Maybe it's because of the customs?
When I got my customs for the first day, I created this really strong 'vacuum' with them that it sounded really weird!
After that I broke the seal and let a part of the 'vacuum' go away and it sounded much better and more normal
A vacuum means the seal is already there or perfect. Don't break it.
Whenever the seal is broken, it would seem very obvious (like part of the sound just leaked away).
And thank you all for the comments. I have visited The Hearing Centre/Ace Hearings yesterday - however the audiologist could only pinpoint slight hearing loss (optimum values were 0-20, I seemed to have 1 or 2 40's and 50's @ 8000Hz). I wasn't too worried about those though - because I have 0 problems with lectures (maybe slight problems with conversations, always needed to ask people to repeat at least once), and of course my music.
One of these days I will be visiting a ENT (Ear, Nose & Throat) doctor.