Certainly, replacing the K-400 with a K-510 and a good 2" compression driver (probably a little better driver than a K-691 if you value the highest octave performance especially highly--10-20 kHz).  It turns out that the Khorn bass bin performance can be extended a little (the black trace in this Jubilee/Khorn bass bin comparison plot):     and the K-510 can be stretched a little on the low end (K-510/K-69-A horizontal polars first):     and vertical polar