i would burn them in for at least over night, my m50s sounded bad when i first got them. they also sounded bad after a months of me not using them, but sounded like they sould after i reburned them it.
so yea burn them in they sound good then
also I've used my m50s on a couple of different sources and they still sound great and spotify from what i heard has great sound streaming. If the off chance they still sound bad, i would try a different audio source, my galaxy s3 sounds better than my pc.