Been using my hd 6xx for a couple of months then decided to sell it.I can’t live without the 600’s midrange and i found this guy he sells a lot of 2019 open box Sennheiser headphones(not gonna tell anyone 
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vs 58x vs 6xx vs 600
The driver in the 58X just cannot resolve like the 600 and 650 can, and those two can’t resolve treble like the 660S can. The 660S is way overpriced but it does extract more detail regardless of its tuning than any of the Senn headphones below its price point.
Sorry for my english this is not my first language