Hi Ian,
I only wonder, may be, mid-bass (doggy box) with the back box wall would produce a kind of acoustic duct, so a kind of higher frequency box resonance can occur as such duct would divide internal volume into two smaller boxes. May be when the ports are somewhere in the middle between these boxes, such influence is less pronounced. All these are my speculations, and it would be interesting (for me, for sure) if someone nearer the JBL technical older stuff can put some "light" on such issue.
Measurements with 2242 drivers, in the box of about 220Lit, tuned on 26Hz with two ports near the bottom plate of the box, are shown.
These measurements are from (in) one vent . So the combined response bass+ driver would be as shown.
Interestingly that AkAbak simulation would predict such behaviors.
regards
ivica