I found mine improved just a bit from the right out of the white box sound after about 20 hours of some pink noise.
The debate between the Shure 840 and ATH M50 has been going on for years already.
I could tell you my take on the two, but what's the point.
I suggest you buy one of each from a retailer who takes returns, there are many of them,
dozens at least. Listen to them for a week or so and return the ones you like less.
Otherwise you'll read the honest but differing opinions from now until whenever; you'll
not get a definitive answer - especially with regard to how they fit you.