I own both MMGs and (non-Meta) LS50s. I won't go into the MMGs, you already own them. LS50s are monitors. They are very revealing. And not very forgiving of bad source material. They will never sound as 'big' as MMGs, but no stand mount speaker can. In their own way, they are almost as room-critical as MMGs, and if used with a sub it better be a good one. REL T/7 or one of KEFs own for example, crossed over at 60-80 Hz. They also take a good long while to break in. New, they sound very lacking in bass, but then one day, the bass opens up and the overall presentation relaxes significantly. And you realize just how special they are.
My experience with the BRXs is much more limited, My impression is they are much fuller sounding than the LS50, but with all the edges softened across the entire frequency range. The amount of cabinet surface area give to passive radiators means that a lot of bass and midrange is coming from the cabinet and passive radiators compared to the very 'quiet' constrained layer cabinets of the LS50. My theory is that is why the overall sound is less precise. Still no one would call it anything but a very good speaker. If I could only have one pair, I could make a good case for the BRX, but would probably step up to the KEF R3, which has most of the LS50 attributes in a more forgiving form, and better bass extension. Since I do have both, I get to appreciate the MMGs and the LS50s, each for what makes them special.
