Since I am not a mastering engineer, i have little to suggest except the pieces of equipment I've sat in on during mastering sessions. I do know that a popular mastering platform is Soundblade and that seems to be the standard in most houses I've worked with and I can't say I've seen anything else in use. For speakers, try looking into Duntech Sovereign, B&W (especially the nautilus 800 series), Augsberger, Legacy...I've worked with the Sovereign locally and those are amazing speakers. However, none is more important in mastering than the setting of the mastering suite because the place needs to be dead accurate for all of your gear to make a difference. Good quality converters are a given and some I;ve seen in active use are the stuff from Prism, Lavry and Lynx.