I have an AT150MLX mounted on an SL1210M5G w/fluid damper feeding the MM input of a Cambridge 640P. I didn't install them together all at once; there was about 2-4 wks between getting the 640p and AT150MLX.
At any rate, in my system, both components sounded extended and musical right out of the box. The 640p was a little bright and edgy I suppose, but both were totally listenable from the beginning and sounded very musical and sorted out within two weeks of playing about 4 hrs/day.
I suspect something else at play here besides tough break-in, and the diagnostic suggestions so far should be chased down, esp. VTA, VTF, etc. What headshell are you using, and are you using the additional counterweight? What VTF are you at so far?
If you suspect your cartridge leads, maybe you should get a new (better) set and try again. You can get silk-wrapped OFC Litz leads from LPGear for $15.95/set.