How to make a full convertible hit hard

So I have a 2001 Jeep Wrangler hardtop convertible that I’m trying to build a box for that would sound good with the top off. My customer wants to go ported but as soon as we remove the top the bass is gone. Is there a certain way I should be facing both the subwoofer and the port? I currently have the sub down and port down and it sounds extraordinary when the top is on! I’m running a Soundqubed HDS 3.110 inch sub At 1.5 ft.³ internal volume after displacements ported at 34 Hz and a port area of 24 Square inches. Anybody have any suggestions?

ghost drive it in to a wall with all the high end equipment you can put in a Cheap Wrangler at 60mph. It'll hit hard then. Ha! 

The reason why is there almost no bass is because if the roof is on, there is cabin gain, if the roof is off, there is no cabin gain. So if you want to make some bass without the roof, you need a driver with huge X-max which can work well in a big net volume box tuned somewhere between 20-25 Hz like for example the box I've designed for the for the GZNW 12XMAX woofer: Box for Ground Zero GZNW 12XMAX (speakerboxlite.com) . Here on SBL on the Graphs tab the cabin gain is basically the interior transfer function (the car icon). Turn it on, and you will see the difference between the "roof on" and the "roof off" situation.

I have a similar scenario. Would the placementof the sub, in my case under the armrest-as close as possible to me, improve bass?

I have a 10" sealed box in the trunk which I'm looking to replace with a build of a vented enclosure and Dayton DCS205-4 8".

Any help is much appreciated. This entire building world is new to me and I'm loving it.

Cheers, Alvaro Sanchis

Please describe in detail the following. Box placement in vehicle and relation to boundary that is removed. i.e. the hard top. Is it Wrangler 2 dr. Or Wrangler unlimited? I think it will be impossible to get the sub to perform identically with top off as with top on, however I do think you can get it to perform well in both cases. I would start by designing it with the top off. In this case you will need to either place the drivers and ports to load off of the floor of the vehicle or be hornloaded by the enclosure. Otherwise the output from both will not be transferred without losses to the listening position by the cabin having boundaries.

Might consider relocating as close to listening position as possible. 2 under seat subs or center console sub or a combination of all.

I struggled too with a soft top convertable (eclipse spyder) and just threw a lot of power at it until it played well with the top down, although top up or down i had to compete with a lot of road nose....overall it was a challenge 

Down firing the subs behind the rear seat is wat I did in my 06 wrangler with 2 pioneer premier 10” dvc subs with 1000 watts pushing them and they beat with or without the top on.