I should of define what a MBM (Mid Bass Module) is and who needs them. A mid bass module will cover the frequency range of 50-250 Hz. Why add a MBM? For some small rooms, there will be a null in the frequency range of the MBM. This is not correctable by EQ which is mainly used to bring down peaks and raise small dips. MBM will provide more chest thump and club like music tactile response. YYMV with employing a MBM so, they are not meant for everyone and may or may not be a good addition to a HT or music system.
MBM are usually made with a 12 woofer. A subwoofer can be used but, cutting it off at 50 Hz is giving up a lot of what it was made to handle in HT. A MBM is placed different than most subs. These modules are commonly used nearfield with a delay to be in synch with the subwoofers. This is not a process done by ear and REW or Omni mic should be used or some other measuring system.
It should be noted that many systems with adequate subwooferage may lack the chest thump due to a lack of mid bass caused by SBIR(speaker boundary interference). This SBIR can cause large dips or suckouts in some rooms. This can cause destructive interference between direct and indirect sound waves in the room. So, it is possible to have high spl at the MLP and miss the chest thump. In an ideal world, one would place 4 subwoofers at the mid point on the 4 walls, followed by on sub in each of the 4 corners as a second choice. I can't overemphasize that a truly optimal HT will have subwoofer in the front and back of the room if the wall mid points are not available.
As a closing statement, we don't see many people on the forum complaining about the 8000 to 25 kHz range where there are few audible modal problems. I don't get caught up on which speaker is better or what avr I should buy because Klipsch has many offerings and there are an abundance of quality avrs. Proper setup is the key to making or breaking a system no matter what dollar amount has been spent. You can't buy quality sound without understanding what is going on in the room!!!