Every sub is different! Give it time. However, I recommend that when you do the calibration, that the sub not be present. So let the underseats play all the way down. Then after the calibration, hook-up the sub. I recommend you set the amp HP/LP at 80Hz, 24dB/oct. Also, you may want to switch the polarity of the sub!
Mobridge DA2 > JBL MS-8 > JL Audio HD600/4 > (3) Hybrid Audio L3SE & (2) Rainbow Cal25; JL Audio HD900/5 > (2) Morel HCW-10 & (1) IDMax 15