Review on Sennheiser HD 660 S: Ultimate Hi-Res Open Back Headphone for Audiophiles by Wiktor Niekrasz ᠌

There is Massdrop58 / 650 is the absence of a muddy and slightly raised middle in the region of 2-3 kHz, due to the fact that they removed the overlaps made by the engineer in the 90s in order to pull out the frequency response, at the same time the hump on the midrange went away, there are also Beyerdynamic DT250 / 250 and Etymotic ER4, this is precisely the evolution of the 600/650 models, and not marketing, and it makes sense to take them.