SoundChoice
100+ Head-Fier
I have the S8 Pro, but have not tried the S8F or any others. My take is that they are super detailed, but musical, with nothing out of place. The perception of soundstage isn't massive or oversized, but accurate (I have years of orchestra conducting experience, and can easily locate the instruments spatially on good recordings with the S8P). Mids are recessed only enough to keep out of the way of the bass and treble without losing content. Bass, especially sub-bass is "real," punchy and visceral. Depending on tip and seal these can be real bass monsters (Spiral Dots, Comply), clean and clear with tamed treble (Final Type E) or unyieldingly accurate (Symbio W).
If I'm looking for a spacious, grand scale listen, I'll grab my Oxygens, but if I want to hear all the nuances of the music (flaws included), the Fearless S8 Pros are my choice every time. If it's a question of on-stage monitoring, it's S8P without question-- aside from the dialed-in EQ, isolation is awesome.
Now, from what HBB says, the S6RUI and S8F are going to be more your thing if you're into hyped and intense maximal detail listening, and S6P for a more typical "HiFi" curve with lean bass. If you've ever been on a large live stage using IEMs for monitoring, you'll understand what I mean when I say the S8P is "stage neutral" not "studio neutral," that is, a slightly elevated bass from ruler-flat that enables the performer to better locate the beat and still get a real sense of what's happening in the room. They aren't everyone's particular brand of EQ, but they're definitely mine; I've never been so satisfied with an audio purchase.
Could you rec for onstage monitoring of male vocals? Or something else?