I pulled one of those prebult sub woofers that go with complete systems, out of the trash not too long ago.  It was a powered sub and was pretty crappy.  I think the sub in it was 7 inches and i had an idea... Why not put it in a scaled down DB?!  Well with some left over mdf i made one and is in my room right now with my rooms stereo.  (i still have the amp that was on the original sub powering it).  You can feel this thing shake the walls in my basement (my room is on the second floor).  Its amazing.  No one that comes over and sees it believes me until i pop the top off of it and show them the "guts".  Well, i have it under my desk and it really gives a lot stronger bass when it is under my desk.  (its a corner desk that is an L shape and the DB is in the corner of it).  But when i move it out from under my desk the bass isnt nearly as strong or noticable.  I tried placing it in another corner of my room but i dont get the same strong bass.  Could my desk be creating a DB boosting zone?!  Or is it just luck?

             -Geoff

same effect here. the desk formes some sort of a horn maybe.
in my case it boosts up between 37 and 40 hz.