I have built speakers with every known form of loading. Bi-polar cone speakers on an open baffle, and especially a narrow one, are a really bad idea.
This is the sort of thing you are fighting.
I love the poop comment on this graph.
I realize there is better Eq, to equalize the appalling FR that results from this technique, but the Eq required is colossal and has to increase driver distortion.
If you really want a dipole it requires a planar speaker, and the best is Quad and it has been since 1957.
To make a fine speaker you have to start from sound engineering concept, and an open baffle moving coil cone driver does not come close to fulfilling that concept.
I think over time I have loaded moving coil drivers in every way known to man. The very least promising and satisfactory in an open back MC cone driver.