a tiny bit of damping at the aperture's top can affect things a lot - small protuberances as K15's shelf can work and offset how much HF reaches the upper half of the coupler
x15 is "bonggy" with C12CX - maybe less reflector curve would help - not sure about more height - how does coupler width vs cone diameter enter into the picture?
slit-vent version of K12 can be highly damped - sometimes too much so and under certain conditions may not play as full below cutoff as lumped vent (?)
whats the k-builder feel for this stuff? - this is beyond math and measurements - has to be from "ear-brain"
heres k12 & x15 to show reflectors
1958 K12
~1966 X15
