Dweaver, great observation and YOU BEAT ME TO THE PUNCH!!
I mentioned a few times around here that I had noticed an anomaly w/ the MD's as well but needed more research. I was fortunate enough to have given in depth listening to 2 different MD's. The first set I used my former typical burn-in process using my SQ playlist increasing volumes each day for 3 days. After extensive listening for a few weeks the mids eventually would become way too forward, loose and bloated on certain songs. I then received another pair of MD's and decided to experiment using the same burn in process but reducing the volume levels w/ respect to each day of burn-in compared to my prior procedure. I was wondering if excessive volume levels or potential clipping had adversely affected driver burn-in or perhaps damaged them. Using this new procedure on the second set I have the MD sound signature but have yet to experience that previous midbass bloat/encroachment issue. The treble also seemed warmer at first than the previous MD's which correlates to your findings on the Coppers. At this point I feel totally comfortable confirming your findings that how you burn-in your drivers can affect the sound signature. Whether the previous MD's had the signature shaped different or the driver got damaged I can't say. I can say they did sound different.