Got something to think about.
One idea is a over sized precompression.
It just something to think about.

the other idea is a change of the layout of the WO

On fostexs web site they talk about the double bass reflex and some science and required sixth sense is needed to get somewhere.
Sometimes thing that should'nt work do! But you can't go wrong
by following all the rules.
can you double reflex a WO? I wonder what that would look like
or a wo with a built in precompression chamber.
As Hulk Hogan used to say" What are ya gonna do brother?"