I think Richard was on the right track - roll the bass off for the mids and highs, which won't be affected by frequencies they can't reproduce well anyway, and use the electronic unit simply to adjust gain on the bass frequencies to suit the output of the mids and highs.
Nice to hear of other 4 or 4A owners - pretty thin on the ground today. I've owned mine for many years and they are just too good to sell despite my having two other systems more current and also giving excellent sound.
My Vandersteen system is over top 1990s vintage - amplified by a pair of PSE Studio IV monos on the bass, and twin mono Classe DR-3 VHC (at around 110 lbs each), biwired for mids and highs (with Classe DR7 preamp and Classe DAC1 D/A). A 'high Classé system ;-)