Before splashing any cash, have you worked on the positioning of the speakers in the room - bring them out from the wall, toe in/straight ahead etc? Have you tried different listening positions (without getting too kinky)? What are they placed upon - stands, bookshelf, table etc?
What is the room like? Tiles, laminate flooring, carpet, parquet? Are the walls plasterboard, brick, wood, etc or is there something like a bookshelf to deflect and absorb the sound a bit?
If the room is causing problems it will for any replacement too.
Now I'm going to pre-empt any answers and recommend the Kef Q100 or Q300. Since the new models came out the old ones are a steal if you can find them. Gives a flatter, darker sound than the MA. No idea if you would like it or if it works with the Sonos! Find a dealer and give it a whirl!