sounds like the same problem I was having with my lvl3 with lvl4 softparts. I figured the spiders just got weak and caused the sub to bounce around and bottom out. I ended up getting pissed off and switching to another brand of sub
I ordered a DC 12 sub over a month ago and it finally came in, but I forget what i ordered. I dont know if it is lvl3, lvl3 with lvl4 upgrade or lvl4.. how do i go about figuring this out??