I don't think the M50 is overrated at all. Maybe I haven't heard the blue box version of the M50s and those are worse than the straight cable white box versions I have but I think they're worthy of the praise they get. Recessed mids? Sure, but they're not garbage and I rarely find the M50s to be at all sibilant.
The Schiit Magni isn't going to be the kind of warm, lush sounding amplifier you're looking for as it's pretty similar to the O2 in terms of tonality. Tube amps are pretty much known for their warm presentation.