Alright, reporting in...
HD 650 are definitely not broken.
After a couple of hours of testing and comparing:
Music like The Cranberries sounds pretty damn good with the 650. Rock and Metal music seems a total mix bag, literally: it depends pretty much how it's mixed.
Chinese Democracy plays pretty ballsy, Appetite for Destruction very thin and unimpressive. Ironic, I know.
Diablo's "Icon of Flesh" sounds ball-droppingly heavy, bass included.
Clapton: Layla unplugged, Ok, I was pretty close to crying here, sounds so screwing good.
But I listen to a lot of synthwave, retrowave, darksynth etc. electronic music and it sounds pretty much all unimpressive. This is why I thought the 'phones might be broken. Somehow the lower end seems if not totally missing, wholly unimpressive and I can't make out the details I could with the 518. This still puzzles me because I'd think even with clearer or cleaner emphasis headphones you shouldn't totally lose some parts of the soundscape. Then again the theme from 'The Expanse' plays flawlessly including the lows.
I now also hear clearly the shortcomings of the 518, since the 650 beat them on pretty much everything but the hit/miss bass. So, can't go back to those!
Anyway, before this turns into a novel: thanks to all that assisted and offered their opinion on the matter at hand! Greatly appreciated.
PS. I went from one set of "These are great!" headphones to two pairs of "These are OK, but..." headphones and it only cost me 435 €, isn't HiFi great!