The reason for plugging port is the port can muddy the bass. A bass note is first sent out the front of the speaker then a delayed signal is sent out the rear port, hits the wall and arriving slightly later to the listener. The port is also 'tuned' to add a bloat sound at a certain frequency. Much like blowing air into a wine bottle and can be tuned higher or lower depending on volume or length. A cheap way to produce bass.
It's important to understand that if you seal the rear port completely (plugged) the excursion of the driver will be negatively affected. THIS IS NOT GOOD! This imparts a restriction on the driver and reduces the overall output of the speaker including the upper bass region. Even foam plugs or socks stuffed in the port will reduce excursion.
An old method that lets the speaker breathe is placing a bundle of straws in the port. It disrupts the tuning of the port so the cheap sounding 'one note bloat' is eliminated yet still allows for full excursion.
If you then properly integrate a subwoofer the low bass should be greatly improved.