My 4 18's have 450 square inches of port, 24" length with around 34 cubes after displacements. It gets down on 26hz, make ya feel like you're floating in the seat. First few burps to find my peak were 158's @50hz and 157.4 @34, didn't test lower but it didn't seem any less violent at 26hz. That's not full tilt either. That was with a old deck with my phone as source, I just got an 80prs to get it all tuned up proper.
Adding length to the port, adding pvc to the inside/back of port as a roundover, or taking away port area would all help with the lows. Lows and metering are not usually done together. I'd build a plug for the demo's/lows where 1db is hardly noticeable, then pull it out to meter.