I heard great things about the System 6000. ($10,000)
But if all you want is to master a two track bus, the extra features of the 6000 including surround sound appears to be an over kill for my needs. I have not seen much more on it that would justify me wanting it on this time. Though the George Massenburg EQ is nice.

On a cost that is in more of our price range, the MD3 and Waves Mastering seem to be pretty competitive. Has anyone tried both and compared the two?