@Shanling
I have two M0s, one bought months back and a newer one given as a Christmas present to me just recently. The new one is running 2.3 and the older one 2.2 firmware. There are physical differences between the two, the newer having a darker Titanium color than the other and the corners a bit more angular. That's not where the differences end, though. I've noticed the newer one having more muddled sound than the first. I've done a comparison between the two using the same music files and with the same headphones. The newer one has more bass but simultaneously more congested as well.
Is the sonical differences that I'm hearing due to firmware, burn in-related or did you alter the sound signature with newer M0s? The difference is pretty obvious. I must say I like the older revision's sound characteristics a lot more. The newer one sounds considerably more congested as I stated before. Will burn-in help some?