I have auditioned the MT50 driven by an Onkyo along with B&W ASW610 sub in a 14ft x 10ft room and it was extremely good for the movies/gaming and decent for music.
Haven't heard the MA BX series but I have auditioned the MA Mass 5.1 package driven by a Denon in a bigger room and it was also very impressive with movies and gaming.
I would say that for dialogue clarity and high frequency sound separation the B&W system was better than the MA system.