This comes from Mr.Flamboyant over on termpro
"imagine a BIG "SPL only" type of box.... another name for SPL only boxes are "one note wonders"....
the ABC box actually has 2 different tuned frequencies.... where the outputs at these frequencies is quite intense.... certain frequencies ONLY come from the smaller chamber, and 1 port (the upper bass frequencies).... which allows those them to be very "tight" and accurate, which can sound very good....
and on the lower frequencies the sub will use the entire box (both chambers combined), and bass will come from both ports....