I don't really feel this is the case. I think the HFI-580's do in fact offer a better sound, while still being in the same sound signature. Both have somewhat heavy-handed bass and treble, but the difference is that the HFI-580's have less recessed mids. (Actually, the mids aren't just "less recessed", but are quite nice.)
And while the HFI-580's have more bass with bass heavy music, I also feel they are less bassy with music that isn't bass heavy. The M50's, to my ears, added bass that wasn't there, not the case with the HFI-580's.
Yep, agree. And 580 has better soundstage as well.