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Joe,
There is NO difference in "dB's" at all, with regards to which port you plug. Zero.
Now, there IS a difference in total headroom above the tuning point with each port you plug. A PB12-Plus/2 (formerly known as the PB2-Plus) is set to 25Hz with all ports open (figure flat extension to around 20Hz in most rooms), 20Hz with one port closed (you'll be linear to around 15-16hz then) and 16hz with two ports closed (call it 10-11Hz... corner loaded, mid-sized room).
The bill to be paid?
Figure about a 2dB drop in total headroom (the ability to handle a transient, or more dynamic range) above the new tuning configuration. Given how much headroom this sub starts with, most folks don't consider the 20Hz setup a compromise of any kind and like the combination of depth and power. So, while you drop a bit of "headroom" when you use one plug, and more with two plugs, you gain power down low.
Physics, as they say, is a PITA.
Keep in mind it's critical you match your port config with your subsonic filter settings.
Drop us a line if you need more detailed help.
Ron Stimpson
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