I just got a new pair of m 80 from amazon. I was listening to it everything is good. I like the sound wish it had a little bit more bass. I streched the headband a little and now i can hear something is loose in the left earphone. Sound is normal to me but When its off and i move the headphone around i can hear something moving inside...Im so scared i think i broke something.
Ask customer support about it, they'll get it fixed as soon as possible.
The bass can be a bit light if you're used to headphones with more! But it's of fairly high quality, I just wished it extended a little lower into the sub bass. It's still pretty impactful though. Many people actually feel it's too much.