My apologies for the intrusion, and kudos to Gene for the excellent review, but I just wanted to clarify some things regarding the MBM. The purpose of the MBM is not to handle frequencies above 80Hz per se, but rather to handle frequencies from 50Hz up to crossover with the main speakers (which is usually no higher than ~80Hz). The general idea was to have a 2-way subwoofer system where one uses a large true subwoofer to reproduce deep bass (~16-50Hz), and then uses the MBM to reproduce mid-bass (~50-80Hz). In many rooms, the best place to reproduce deep bass is a different location than the best place to reproduce mid-bass. Also, the driver design requirements are different. The MBM useage has some flexibility too. As a single unit, some people use it in the nearfield (for higher direct-to-reflected sound ratio, higher impact, lower distortion), and others use it next to the true subwoofer (for lower I.M. distortion and wider dynamic range). In pairs, some people use it to flank the left/right mains, others use it on each side of the couch, and some people even use it next to multiple true subwoofers. When used in pairs, one may be able to use the MBM's from ~50-150Hz, depending on where the MBM's are placed.
I do agree with Gene that adding a second identical true subwoofer can make for a big overall improvement. I recommend this all the time. The MBM concept is not mutually exclusive to this. In fact, some of the finest systems I have heard include both multiple true subwoofers and multiple MBM in the mix! Note that with very large and/or very expensive subwoofers, adding a second identical true subwoofer may not be practical nor affordable, so the MBM may make some sense for these people. That's why we've had owners with PB13-Ultra, Conquest, etc who have had great results adding the MBM. On the flip side, in some cases it may not be practical to add the MBM. For example, in some instances (even within our own lineup) I would recommend dual true subwoofers instead of one true subwoofer and one MBM. There are even some instances where I would recommend one large true subwoofer instead of two smaller true subwoofers. We would need to evaluate each situation on a case by case basis to determine whether or not it makes sense to go one particular route vs another.
Sincerely,