If your system seems to lack that "mid-bass" punch, you may need some MBMs (which is short for "music-bass modules" or "mid-bass modules").
The GSG MBM's are designed to complement the Marty Sub line-up aesthetically. The MBM-18 has the same width and depth as the Marty Sub line-up as well as the same height as the Marty Cube. Of course, for the 21" we had to make the cab larger, but it still has the same design and easy assembly process.
The MBM-18 is tuned to 27Hz and works great with a number of pro-audio drivers (Dayton PA460, B&C TBW100, etc.)
The MBM-21 is tuned to 28Hz and kicks up the output. It works great with a number of 21" pro-audio drivers. At this time, we recommend the B&C 21SW152 or the 21DS115.