My HD650s have just been away to the Sennheiser doctor. The right channel failed, and they have been repaired under warranty. I received them back in the same box, but they look newer... Maybe they went to the Senn health farm after the Senn hospital - they don't just look new but the sound even sounds livelier. Where is the serial numberon the actual phones these days?
Not sure the HD650 are even serialized... are they
They may have simply given you a new replacement? But even if they did fix it, they likely just switched out the old drivers. In which case, you might have a new pair of drivers (considering they are said to be matched) that have yet to be broken in fully yet. Slight sound variations from pair to pair wouldn't surprise me. LCD2 graphs indicate that to be the case. No pair is identical.