Note: Now I know the sub bass extension isn’t a JBL but if I had a choice I’d change the sub bass driver over to one of the newer 12” JBL high powered sub bass drivers.
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!