Don't go under 6 cubic feet. Anything less generates an underdamped response curve, whether sealed or ported. Basically, if you go under 6 cubic feet or tune above 30Hz, you're going to have a big midbass peak.The PiAlign recommendation of 6.25 cubic feet tuned to 25Hz ensures the best performance this driver is capable of in a small box. I used to make Premium Studio Series four π speakers out of them, and plans are still available if you're interested.