I think both the O.P. and I would appreciate some learned commentary/opinion on these statements excerpted and quoted, in the context of sub and mains integration, from Barry Ober (Soundoctor). Personally, I am not knowledgeable enough to judge what he says, so I am a neutral observer here.....
"Lets examine ported speakers. We'll start with the worst case, the port in the front. At mid bass frequencies, say 50-80 Hz, the LF driver moves IN the cabinet, the air in the cabinet is elastic, and the port air moves out of the cabinet. Because of the frequency at which the cone is moving, by the time the cone moves out (forward) again, the port air is now moving out, so in front of the cabinet the two air pressure sources sum together and you get a fake bass "bump" or "boost".
As you go lower and lower in frequency, at some low frequency the air pressure from the LF driver and the air pressure from the port are exactly opposite each other, so they cancel, and there is no more audio at that frequency: it disappears.
When the manufacturer of a speaker cabinet defines the frequency response (i.e., 37 Hz - 20kHz +/- 3dB) this is what is defined by the entire arrangement of the port and the air in the cabinet and the driver. At some low frequency the port air is exactly out of phase with the driver air pressure and since they cancel, there is NO output from the cabinet into the room. Therefore with a ported cabinet, the entire sloppy concept is this juggling game between the response of the drivers under air pressure, the passive crossover inside the box, the port size and placement.
That means IF you were to simply put those same frequencies through the mains and the sub (that means with no crossover, and this is the mistake that nearly everyone makes) you would now have 3 sources of LF energy and differing phase: the 'main' LF driver, the port, and the sub, all fighting with each other in the time domain. A further corollary is that since the air inside the [mains] cabinet is elastic, the phase relationship of the port air to the driver air is also a sliding one; that means it's "out of phase" — and smearing — over a wider range of frequencies than you might think.
A ported sub for home use is even more wrong than ported mains. Now you would be attempting to acoustically add together in the room at least SIX low frequency sources with differing phase and frequency slope conditions: the LF drivers in your two mains, their ports, the sub driver, and its port.
If your speakers are ported, you SHOULD close (seal) the ports......What you are trying to accomplish is to NOT have multiple sources of differing phase relationships (the main driver, the port air, and the sub driver) at or near the crossover freq."