I have seen quite a few speakers that come in either a MT or MTM arangement. They add a second driver to the MTM. The second driver a mid/base is the same as the one in the MT. So essentially you have 2 mid/base drivers(of the same type) and 1 tweeter. The crossover is the same it's still a 2 way. What benefit can be derived from this that you can't get from the MT with just the 1 mid/base and the tweeter?