Hello, I have a new case for my GNZW 15http://www.ground-zero-audio.com/deutsch/05_nw15.htm. Currently it is installed like thishttp://www.ground-zero-audio.com/deutsch/06_nb3800sxt.htm The dealer said Vollgenders. I should build a box that has about 60L +/- 1L volume. Then use the subwoofer and use a passive membrane opposite the subwooferhttp://www.ground-zero-audio.com/deutsch/05_p15.htm No bass reflex tubes etc. That’s what the dealer said. Now I wanted to ask you if he just wants to talk me up with a passive membrane and a lot of work or is there something about it? Greetings Hermann