Almost sounds like the sub reaching it's excursion limits, which is very possible in a DIY sub reaching down to 10Hz w/ boost. Test the sub, when you hear the "womping" sound, back the gain down on the sub amp to see if goes away. If it does, your subs reaching it's Xmax.
Also, using boost to compensate for a lack of power will toast your sub. It'll start clipping without any warning. I've seen Tumults in 4+ cu ft sealed reach 8Hz at 100+ with over 600 watts.
Check out this one...
http://www.hometheatershack.com/foru...t-project.html
I think you can produce the same results with the right power and a bigger box.