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 > ARC XDi 600/4 > JBL Gti 408 Mids & Vifa XT19 Tweeters; PG Ti2 1600.5 > Morel HCW-10 & IDMax 15