What model SPL meter do you have? You may be using the old correction factor for a new meter, which would cause more apparent bass than reality in your curve at those low frequencies.
See correction factors here:
http://www.hometheatershack.com/foru...downloads.html
Regarding bottoming, I haven't heard it personally, but from what I hear...if it happens, there is no question it happened (clank!).
Is your sub in a corner?
Pete