Pay less attention to the port per cube, and look more at what the vent velocity will be with __ port dimensions. I like to stay between 24-28 m/s after I factor in the ssf/lpf. I did a box for 4 sa-12 that only had 13sqin port per cube, last week I did one for a custom 10" that had 23sqin per cube. The amount of Port per cube isn't even relevant to anything, the numbers don't matter, the vent velocity is what decides if you have issues. Too low and the subs perform like shit, and too high and you'll get port noise and the subs will still perform like shit.
Edit: I'd stay between 2.5-2.65cuft net per sub and they get loud.
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