Porting of cabs is a way of extending the bass response of a driver unit compared with having the driver in a sealed cabinet by allowing some of the low frequency sound to be projected from the port.
It is important to have the cab tuned to the resonant frequency of the driver units so that the sound projected from the port is in phase with the sound eminating from the speaker otherwise you will reduce bass response!!
To accurately tune the port you need to know the Thiele Small parameters of the driver units you are using. It is very rare to find these for guitar speakers. Furthermore these parameters are unique to the drivers being used, so if you want to change driver units in a ported cab you will effect the resonance of the cab.
I'm not 100% convinced that TS ported cabs are much use for guitar. There are several problems with them, not least you probably don't know the TS parameters for the speaker. Below the tuning frequency of the system the bass response drops like a stone, and the loudspeaker cone is unloaded, which can cause it to travel excessively and be damaged.
Having said that there are several ported or tuning guitar cabs out there that have good reviews, eg the Dr Z 2x12, and the THD 2 x 12 (although this a rear slotted port so is really a "semi-open-backed" cab).
For bass guitar, where increased efficiency in the bass frequencies is beneficial, a ported cab is probably to way to go.
