I think you should just listen to them and not worry too much about burn in. But the M50 is a nicely built headphone for 100 dollars and is a nice relatively balanced sound. I don't think the 280 is a good headphone for music it is very laid back and sounds almost veiled or muffled to my ears. Very boring sound. A good fun portable headphone to start with is the HD25-1 ii.
11/30/12 at 10:40pm