I remember several models of the MB Quart lineup, but not any specific models....this was sometime ago. They were ok if memory serves.

Looking at the specs you listed, a couple of things jump out:
1) they are very inefficient (87db - M80 listed at 98db)
2) crossover points located in difficult frequency points - although I would suspect the bass is again crossed over to utilize the dual woofers.

Are the MB's up for replacement?