Skylab, I can't answer your question, as my experience is based on the PRO4 AAT only...but I'm not very surprised to ear that after 50H of break in, there's a serious difference in sound reproduction.
All I can say is that, if the MV-1 reacts like the PRO4 AAT in terms of break in you should get futher improvements in the next...150 to 200 hours.
But, I'm not sure the PRO4 AAT sounded so badly (your description is terrible !) right out the box...
I'm not very comfortable with audio description words in English, it's not my mother language, sorry, but I would say that "muffled", "canned" or "boxy" should suit correctly these headphones when I first listen to them.
Now (after 250 hours) this "feature" is only a bad memory : the soundstage is slightly wider, it is remarkably neutral with great precision/details in all the spectrum (after all, it has been designed for studio use in mind).
I've noticed an improvement in the bass department and highs are crystal clear now.
Only the weight didn't changed...
I'm not sure that it will be all the same for the MV-1 but, I suspect that sound changes could be surprising (I hope).
Please, keep us informed...