Thanks a lot for your prompt responses, everyone! Lots of useful information already, so let me answer some of your questions and ask about some additional points.
What are your music sources? From what you mentioned you have CD, Bluray, and computer. Anything else? Do you listen to FM radio?
Same question for movies. Bluray discs, TV, or internet downloads? How do you connect your TV or Bluray player to your present receiver to watch movies?
My main music sources are my computer (lots of flacs ripped from my CDs, some of them 5.1 bought online, and a bunch of additional .mp4 from iTunes store), CDs (I have a marantz cd-67 mkii, also from the old days), Blurays(through my PS3) in this order. I do not listen to the radio outside the car. Regarding movies you guessed right: streamed from internet (using PS3 or my laptop), Blurays (PS3 again) and TV.
[...] You may not be familiar with this, but it becomes quite important if you add a subwoofer. And you should.
Do you think the subwoofer should come first than a central speaker, then?
Thinking about it, even though my use would be 50/50 for music/movies I give higher priority to music, at least at the current time. I am leaning towards the AV since the beginning, but I was a bit concerned that I would end up losing quality for music playing compared to a stereo. This seems to be quite a polarizing topic, would a stereo receiver give significantly higher quality for music that an AV? (at the price range I'm considering, preferrably below 1000€)
Thanks a lot, I hope other users also find the discussion helpful.