I bought these as a slight upgrade for my ATH-50. As I had them for a few years and them themselves had been replaced by the ATH-50X. I decided the take the plunge and buy the latest offering from Audio Technica. They're one of the most expensive monitoring headphones on the market at the moment and after reading reviews in soundonsound and future music I was blown away by the amount of positive feedback and reviews they had received. On first inspection they're lighter than my 50s and I imagine this will be very handy for longer mixing sessions. I understand the frequency range is also slightly greater however this is only really for dolphins and cats. Overall I'm impressed on first listen and they're a noticeable improvement on the 50 however I cannot say so for the 50X as I have never owned.
The quality of the ATH-M70 X is very high both for the materials used and the sound quality. It comes with a handy case. They are very precise and accurate headphones and therefore you will be able to detect what's wrong in your production. Therefore they're meant for professional use. The downside is that they're quite expensive.
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