12hz produced directly by a driver at say 90db isnt audible at all. The effects however can be heard further up the spectrum. A speak producing material as say 50 only, doesnt sound the same as one producing 50hz and 12hz together. This is in my own experience though, and the same content produced by only a speaker compared to a speaker and a port also sound different. This is even though the measured in room responses identical as close as is possible.
I did a direct test between the SVS PBU and PCU, both in the same room, on the same system, with the same eq and the same measured response. Even these two supposedly identical subs idnt quite sound identical, and the difference wasnt measurable at all with any of the equipment I had to hand.
http://www.youtube.com/watch?v=bYCB8jmX1pc