I'm not a big fan of leaving phones playing while I'm not there, so I've just used the PK3s normally for the last three days, that's about five hours or so per day.
I can't say if the PK3s have gotten less bright, but they're still clearly brighter than the M2C. It's not a small difference. They don't sound broken or anything, just brighter than the M2C.
I did a quick test with SineGen to see about the bass extension, and it seems, according to my ears, the PK3 may go a few Hz lower than the M2C. In actual use (i.e. in this song -> http://www.youtube.com/watch?v=JLHprjp403c from 1:34 or so), the M2C instead goes lower than the PK3 while still keeping the bass notes fast. I didn't really compare the bass beyond that one song, and even that was just something I noticed by accident, but in any case the M2C did better bass-wise in that particular (YouTube) situation.
Overall, I still think the M2C sounds "more expensive" than the PK3. The M2C has a fuller, more lush sound while the PK3 is (still) on the brighter side in comparison. That's not to say the PK3 is bad; it's not.
Edited by vid - 11/11/11 at 1:34pm