As someone else mentioned the 3 8"ers will have more displacement than the single 12.
It may also be that ML payed proper attention to the mid/upper bass. A lot of what we perceive as bass, including the punch resides in the upper bass. His mains will also be contributing to this.
Lastly, the ML sub drivers may well be more efficient. I did some modling of two drivers yesterday. One had an efficiency of 85db, the other 94. The higher efficiency sub had greater output starting at around 60Hz. The flip side of this is that the less efficient driver had an additional 7mm of excursion and had considerably more output at 20Hz, but for music the higher efficiency driver would have the distinct advantage, partucularly with a higher cossover.
If your friend has the sub positioned for maximum room gain, he can combine that with higher efficiency to produce some impressive bass.
