shove a "nerf" ball in each port,because that is basically what the plugs are.i'll almost bet you won't like the sound,MOST ported boxes,i'm sure there are exceptions,are tuned with # of,length and diameter of ports for maximum output.(along with the box volume (minus the port,driver and bracing displacement))xmax(1 way excursion) comes into play also,along with a bunch of numbers that are useless to most,unless designing a box to a particular driver

i've been(on a hobby level) designing and building sub boxes for a few years now(with outstanding results,i might add)

as soundhound stated,you will lose efficiency,and i don't see plugging ports as a way to get extension,as my ported boxes have all had higher output and lower end tuning than my sealed boxes.but as i said,i'm sure there are exceptions.

b & w supplies "port plugs" to REDUCE the bass extension and ouput to several of their speakers,so you may be reading the whole thing wrong,"dropping the extension" i thing should read;reducing the extension.but this is just my interpretation of things and i could be wrong...