Adrian, a ported sub has a lower tuned frequency than a sealed sub(other factors being equal)but rolls off at a rate of 24dB/octave below the tuned frequency while the sealed sub rolls off at 12dB/octave. Plugging one of two ports has the effect of moving the tuning part of the way toward being a sealed sub. The frequency where the sub begins to roll off is then actually higher, but because the roll-off is slower, the usable low frequency limit is extended. So, more output higher up with both ports open, but more extension with one plugged.
