Ive got L7 subs with my Alpine kit and imo the amp is too powerful even for the L7s. If you listen at loud volume and base much more than the 0 setting then I can fully believe that the base subs have blown. I have my bass set at about +2 bars and at high volumes you can hear the L7 subs starting to distort. I'm starting to think the alpine amp might be capable of driving sws-8's but that a whole other subject and debate!
Check the switch position on your amp. On the underside of the amp which will be velcoed in position. Do you have professional navigation or professional headunit?
E90 330d - N57
Prof Nav - 6FL - Visability Pack - Alpine Hi-Fi Upgrade - L7 Subwoofers - L7 rear mids + tweeters - 6FL