I believe John (JBM) is a bit discouraged about the QC subs. Let me explain and hopefully I can get it correct.
To skip the long version. John and AJ had built this "FOCUS" with 8 of the MASSIVE AUDIO QC12 subs. These subs are being powered by (2) POWER BASS XA4000D amps on a 16volt system. When desting the peak frequency it had beed discovered they the note was at 51-52Hz. WAYYYY to high for a street beater/demo car for the shop. SOooooo, they deciede to go with only 6 of the subs and give each driver 2.4ft^3 and then give the box a 28" deep vent.
Keep in mind, box volume, and face area of vent are in spec from MASSIVE AUDIO.
According to John, the enclosure is tuned roughly around 32-35Hz. Even still, when running a sweep, the peak frequency is still at 51-52Hz. So here's the problem:
Tuning frequency at 45Hz.....................best # at 51Hz
tuning at 35Hz..........best #'s at 51Hz
Now the box is tuned arounf 30Hz, and the best (strongest SPL) is still at 51-52Hz
He even put 2 of the QC in another car, powered by a different amp and in a different enclosure. That car also had a higher SPL at or around 51Hz.
According to John, those subs just don't have strong output below 50Hz or so. I told him to call Massive and talk with Jeremy as there's got to be something wrong somewhere. I've never seen ANY subwoofer peak at "X"Hz no matter what the enclosure tuning is at. It literally defies science and logic.
I have not personally heard the car, but I talk with John on a daily basis. Hopeful;ly he'll chime in and give more info.
He has since (in the last few days) removed all the QC subs and wanting to sell them. I would really love for him to stay with MASSIVE AUDIO, but if they don't work for his application, I can understand.
John, if you can, give more info.
Jeremy, if you see this post, please give some thoughts.