I don't consider myself an expert on cabinet design nor do I consider myself an all unknowing fool. I had a brainstorm and I just thought I'd share it here, flame me, chastise me or praise me freedom of speech is everybody's right.
Ok so here's what ran through my head, a speaker cabinet that has the ability to breathe when it needs to but not without some effort. Correct me if I'm wrong but it appears as though the higher the frequency the less a speaker diaphragm has to move so a horn is like a sealed unit because it moves very little air and the woofer is open because it moves a lot of air. therefore a large port would give the best lows but nothing at mid range and the horns take over above that so it is removed from the equation. So my brain fart is a cabinet with a set of valves similar to a reed valve on a 2 stroke motor one to let air in and one to let air out. This would then allow the cabinet to operate like a closed cabinet when mid ranges are playing and ported when the lows kick in providing it takes some effort to open and close them. Tuning would be achieved by the amount of pressure it takes to open and close the valves. Like I said you'll get a laugh.