It struck me last year when I was toying around with extending the sub bass frequency response when experimenting with carpet tubing placed over the (air duct port).
This time I used a different approach with a small piece of plastic tubing about 7" long and 4" in diameter. To hold it secure over the (air duct port) and to prevent any damage to the cosmetic look of the Eltax R-12 A I used silicon to hold the tube down.
Has you can see it takes around 30 minutes for it to dry off and thou I could easily remove it without any damage what so ever, if it sticks to your fingers it washes off without fuss!
I repositioned the sub bass extension afterwards into a different location. The overall result was an impressive (6db boost) and lesser noise from the air duct port at the lowest frequency when playing hard!
Yeah that's right Frankenstein yes I like to take subs apart and attach new parts to them for a better life. And it looks like this was quite an impressive improvement with hardly any cost.
You should try it out? If you happening to have a sub bass unit that looks like the one I have with a air duct port facing the front, turn the sub over onto one side so that the port is facing upwards and place a carpet tube over the port the longer the better.
I was wowed with an 8 foot carpet tube the depth in the low end was DEEPER than before!
and lesser noise from the air duct port at the lowest frequency when playing hard! 
