The intergration of a ported full range speaker to a sub will be dependant on the correct crossover point and slope in relationship to the tuning frequency of the speaker. The Freq response of the sub and it's ability to play at or above the xover point should also be considered as well. Larger drivers 'generally' don't do well higher up the spectrum with cone breakup being the most common artifact to result. For example, if you choose a crossover setting of 80z, the sub will be playing material above 80z, but at a reduced output. The same goes for the speaker below the 80hz xover point. Smaller drivers usually stuggle with LF content due to the small surfae area of the cone and the limited excursion of the drivers suspension. How much reduction or how far up the audibility would be dependant on the 'slope' of the low pass filter used or in the case of the speaker how low or loud dependant on the high pass filter slope used.
Personally i prefer all sealed for smaller driver systems. It prevents the drivers from playing to broad of a spectrum which reduces power handling and introduces distortion. Crossing well above tune in a ported system works good too. From here it's just subjective or opinion based on listening taste.
Basically what you want to achieve is a Subwoofer taking over the low freq playback at a point above where the main speakers begin to roll off thier low frequency playback without having a hump or null in the response at or around the crossover point. The placement of both the mains and subwoofer will also play a part in the overall response. All of this is best accomplished with measuring tools but with carefull listening and experimentation and adjustments, the same results can be realized.
