It is about this product 
This is one excellent device and I am very satisfied with it. However, for reasons completely unclear to anybody, the manufacturer has stopped just one step short of providing us subwoofer output.
Main speaker amplifier has high pass filter which cannot be turned off so you never can get truly deep bass into your main speakers.
If you go to Settings -> Sound Settings you can see that you can change turnover frequencies but you cannot turn the high pass filter off.
Go to Settings -> Device Port and chose the left one.
You get the full description of all 20 pins. The one we are interested in is marked A18 – SUB.
When you look at the connector you will be surprised that it is not connected. Truly amazing. This is the cable.
There are many cinch connectors on it and you most probably don’t need them all. So you can remove number A19 and insert it into number A18. I cannot provide a close-up picture so here is verbal description:
Use a needle to lift small plastic tab which holds pin in place. Be careful not to break it off. When you lift the plastic tab you can pull the pin out of the connector. Now simply insert it into number A18. Done!
Plug your subwoofer cinch connector into in my case CVBS_IN_1 and enjoy deep bass.
I have ordered this one 
Update: I have received abovementioned subwoofer and installed it under the passenger seat where it fits perfectly and is virtually invisible. Considering its size I am very satisfied with its sound and depth of it. Recommend!