The Net volume you need to use for modelling the G-Sub is about 290l, because you need to account for the space taken up by the drivers, bracing, port and handles etc too.
With the longer port the tuning ends up about 37Hz IIRC.
That, plus the fact that the port isn't particularly big in the first place, means that trying to tune it another 10Hz or so lower isn't going to let you get the best out of most drivers that have any chance of being happy tuned that low in the first place.
Your net volume will get even lower as port length increases, and the kind of drivers that are happy going down to 30Hz or lower tend to want bigger box volumes. If you'll be pushing them anywhere near full power, you'll also want a larger vent cross sectional area to minimise artifacts from vent airspeed being too high, and any time you make the vent area bigger, the required length to maintain your low tuning frequency goes up even more, further eating into the available cab volume.
In short, you need a bigger cab to start with if you're serious about going that low at any real volume. Have a look at 18 Sound's dual 18 vented cab as an alternative starting point: http://www.eighteensound.com/Portals/0/EnclosuresKits/18sound_18_dual_subwoofer_kit.pdf" rel="nofollow - http://www.eighteensound.com/Portals/0/EnclosuresKits/18sound_18_dual_subwoofer_kit.pdf
There's a good thread about building cabs very similar to this over at soundforums.net: https://soundforums.net/forum/low-earth-orbit/diy-audio/11488-no-compromises-front-loaded-double-18%C2%94-cab" rel="nofollow - https://soundforums.net/forum/low-earth-orbit/diy-audio/11488-no-compromises-front-loaded-double-18%C2%94-cab
PS don't worry toooo much if winISD predicts longer vents than in plans - it's often the case that when a vent is built in adjacent to the cabinet wall, it behaves as if its tuned lower than just the physical length of the shelf. That's because part of the walls of the port are extended backwards by the cabinet walls. WinISD's calculated lengths are only valid if you keep the internal end of the vent well away from the side or back walls of the cabinet. If you're not building straight off a plan from a trusted source, you're best to build a prototype with some method of measuring port tuning and adjusting it if needed before building your final box. For example, make one whole side of the box removable (held on with screws only for testing) and use low strength adhesive like silicone sealant rather than wood glue to hold the vent panels in place so they can be removed and replaced with longer/shorter ones as needed until you're sure the tuning frequency and overall response of the cab are what you want.
Good Luck, David.
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