Im looking for an Upgrade for my FH5.
I know you said 500€, but if you're willing to push that a little bit, the Westone UM Pro 50 might fit your needs. They're $650USD (575€), though I don't know if that's the actual EU price.
- I would love it a bit warmer (not darker if that makes sense..).
I'd describe the bass as quite warm, though if you listen to a lot of electronic or hip-hop it might not satisfy. For rock or jazz, the bass is pretty great. The treble is quite articulate, but it doesn't seem emphasized like on, say, Beyerdynamics. It doesn't have a lower-treble / upper-mids emphasis that a lot of 'phones have, which makes the UM Pro 50 sound a little less "exciting" at first listen, though that impression lessens with time. My experience is that the included foam tips caused the sound to be pretty dark (truncated treble), but pretty much any other tips fixed that for me... Everybody's experience differs, or course. With Comply or silicone tips I'd never call them dark, though.
- not as mid-centric and vocal forward.
I don't think they're mid-centric, per se, but the mids don't sound recessed either. The un-emphasized upper-mids / low-treble make them almost entirely fatigue free, at least to my ear. I do think the mids sit slightly below the bass/warmth region, though.
- a tad smother, the BA drivers can get a little hard sounding sometimes. Overall im missing ´charm´ in the sound
What exactly do you mean by "smoother," "hard," and "charm?" Those are all really subjective terms and what sounds that way to you might not sound so to me. The UM Pro 50 are BA and sound like BA (very articulate treble, highly detailed and textured, very detailed bass; not "romantic" like the HD 650), I guess, but the un-emphasized upper-mids / low-treble plus warm bass go a long way in making them very listenable, in my book.
- a bit bigger Soundstage
I have no idea what the FH5's soundstage is like, but I was surprised by the UM Pro 50. In some ways it's actually larger than my open headphones (Senn HD 650 and Beyer DT 1990), but it mostly extends laterally without much depth or height. I'd say depth and height are comparable to other earphones I've heard.
- better fit, as they dont stay well in the ear.
Again, no idea what the FH5 is like, but the Westone are fairly deep insertion with an over-ear wearing style. I've never thought they were going to fall out. I find the included to foam to be pretty uncomfortable, but with any other tips I can wear them for hours. I recently went on an eleven hour flight and using Comply was able to wear them for almost the entire time without much problem.
price: As cheap as possible, maybe up to 500 Euro.
This made me laugh a little. Different strokes for different folks! I'd call "as cheap as possible," like, $50 or less, and "maybe up to 500 Euro" to be borderline absurd. Maybe I'm a horrible cheapskate.
Anyway, hope this helps some. Good luck with the hunt!