Cliff notes:

• Factory (premium sound) 6x9 sub works but is dry-wrotted and sounds bad
• I'd like to fix without removing seat/parcel-shelf
• Thought of shoving a shallow-mount dual-coil sub in the gap to the left of the factory-sub, using original sub wiring (disconnected from factory-sub)

The plan: unplug the old sub and attach the contacts to this 10" DVC 4 Ohm sub with shallow 3" depth:
PYLE PLWCH10D 10-Inch Ultra-Subwoofer B001588ITQ
and then figure out how to mount within the gap-space to the left of the factory-sub (left-side when looking into the trunk)

Two concerns:

• Will the factory premium amp have enough wattage to move the magnet-coil and make this new/bigger 500 watts RMS sub functional?
• Will the upgrade to 10" sub be sufficient to overcome the lack of porting to the cabin between the sub and the parcel shelf? (the motivation being the many other posts showing subs in trunk that are audible in car)

The speaker shelf will have to come out. The factory DVC is an 8 ohm speaker. I doubt that the factory amp can handle a 10" sub and still sound good. If I were you at this point, I'd take it to a stereo shop. They'll be the ones that can answer all your questions.

Is the factory rear sub defiantly 6x9? I only ask because I'm in the process of upgrading to premium sound, and I've tested a shallow 6x9 in the hole and it doesn't fit. I've measured the hole and I'm getting 5x7.
Has anyone got measurements or photos of the factory sub in situ?
S.

It's certainly labeled a 6x9 but I've not measured.

Took it to two stereo shops and both seemed more clueless than me as to if it will work.

The thing is that you can exceed the depth space of the left-side alcove above the gas tank if the speaker-driver is shallow enough - allowing a 10" or even 12" to fit, it will just push back on the trunk liner a few inches but is otherwise unobtrusive.

The factory subwoofer is a dual voice-coil design, so you you can't just replace it with a standard subwoofer.

You will need to pull the shelf regardless of what you do, and remove the 6x9. Doing so will open up a hole there to let bass through if you run a sub in the trunk. The factory amp probably will not have enough power to drive the 10 and push through cabin damping. The 10 lists efficiency at 90db (/1w-m?) which is fairly decent for a car sub, but I'm sure the oem driver will have a much higher efficiency.

Also the qts of the 10 is very low, which means it will have poor low-end frequency extension if you're planning to set it up infinite baffle, in the way that the oem speaker is set up. If I'm wrong correct me, that's how I understand it from reading it will be placed in the gap between the gas tank and trunk liner, firing forward, with I'm assuming some kind of mounting plate/baffle to seal the speaker.

the OEM sub is 6x9, to install it you will need a height spacer, you can try pulling the one off the oem driver, or have a new one cut. see here for an idea https://www.jaguarforums.com/forum/x...thread-168335/

This all being said, the 10 is a $40 sub so it's anyone's guess as to how accurate the specs are to what the sub actually is.

The pyle is dvc too

Very valuable information, albeit discouraging, lol.

So basically, it's unlikely to work well, but will be functional. As far as infinite baffle, your assessment is spot on correct, does being sealed in really make such a big difference if it's already in a very tight confined space? Obviously an air tight chamber is different physics, but I'm looking for better bass than what I have (no sub) with minimal labor for installation

Makes a huge difference if the sub isn't designed for free-air. Even though it's a confined space, it's not even remotely sealed, and there is a lot of air in there between the shelf and tank and then the porous carpet divider into the trunk, etc. Realistically, you'd be better off just putting the money into a decent enclosure in the corner of the trunk, a good amp, and you'll be miles ahead of what that will sound like in the window.

Without sealing one side of the speaker from the other, it won't make much sound, esp with subwoofers. Whenever I build a box I will end up triple-checking all the edges for sealing, even small cracks at the seams can make a noticeable difference in quality.

I agree with Charlzo in that a dedicated amp, and a small box built into a corner of the trunk will sound worlds better, and probably be less effort than trying to seal a subwoofer against the trunk in an infinite baffle setup. For that to work somewhat effectively you'd need to cut a plate to replace basically the entire carpet sheet covering the gas tank, somehow seal around that, work with the trunk lid arms, and so on...

You can fix the foam surround with a kit from speakerworks.com.

Cost is less than $20 and the factory sub sounds pretty good when working properly.
Vector

I'd love to get my hands on a factory sub, but the only one I can find for sale in the UK is £115 currently, which I feel is too much for a 2nd hand unit.

If anyone has a factory sub they'd like to sell me for a more reasonable cost, please get in touch with me. US or UK it doesn't matter!

Vector, lol, I have the kit, bought it 5 years ago and have yet to put it in due to the effort of removing the back seat, thus why I'm going for plan B.

Removing the back seat isn't difficult, it can be done inside 10 minutes and you wouldn't have to completely remove the seat from the car anyway, just tilt it forwards.

If I buy an amp like this:
https://www.amazon.com/s/ref=nb_sb_n...rds=B003GSLE2Q

That takes in 2 high level channels (Right + Left) and then outputs a since mono channel, then any subwoofer can be used right? No need for a DVC?

It is really not that difficult. Follow the directions and be gentle with the components.

You need two people to pull up on the seat back to get it out.

Don't break the seat belt plastic trim pieces, be careful taking them out.

Be careful with the package shelf as well, don't break or crease it when you remove it.

For me, first time took an hour, second time 15 minutes.
Vector

Not that I'm trying to contradict what Vector has said, but I've removed and installed many XJ interiors now, and have it down to a fine art. These days I can swap an entire interior around in under 2 hours.

It's recommended to remove the plastic seat belt bezels first before lifting the seat, a thin but long flat head screwdriver and gentle persuasion is key. The upright part of the back seat can be removed by one person, if it sticks in place a few gentle taps with a rubber mallet at the base will help dislodge it. The rear shelf is held in place by two fir tree fasteners, release them first before lifting the shelf.

Best Of Luck.

Thanks for confirming the size of the speaker for me though, despite all the XJ's I've worked on I've never encountered a Premium Sound equipped car. The Premium sound is a rare option in the UK, I've only ever seen a few cars equipped with it. I'm currently sourcing parts for my own premium upgrade from the USA which is bizarre for a British car.

Short answer, yes, a mono-out amp can be used with a single voice coil sub.

I'll also add, that it does depend on what sound you're going for, and all that. As noted, the rear seat and shelf removal really isn't THAT bad, it just takes longer the first time while you get around to it. I did the subwoofer repair on mine a few months back and took it out in the parking lot at work the first time, to remove the 6x9. I then glued it overnight, and put it back in the next morning before work. I made it harder on myself by not removing the trim pieces in the pillars, but it was still doable. The sound, while certainly not the same as having a dedicated 12" in the trunk, is still better than it was without it by far. Also, it does depend on how you have the factory stereo's sound stage set, as to what sound you get. If you set it to "All" it sounds weaker in a sense, than if you set it to "Front", which is where I now have, and have left it. This will also play into it when you set up an amp off those lines as inputs - if you change the settings on the head unit, it will change things there too.

A wealth of very good feedback - thanks all!

So now the big question is what would yield the best sound results?

-for about $150 total I could add the above linked 1500w amp, a 12" shallow-style sub in a compact 6" deep box that should be small enough to "hide". I've already wired in a switched power source in the trunk, so this should require under an hour to install, the majority of effort being Jerry rigging a mount to secure the box to the car.

- for a few hours of labor I can replace the bezel on the existing dvc sub, with the risk of additional time + $ if plastic trim pieces break (which there are several reports of that happening in other threads)

Fyi, my life "situation" is that I'd happily spend a couple hundred $ to avoid a few hours of install labor...

The dedicated sub would probably sound better, esp if you're trying to avoid pulling rear seats. Even with that I'd still want to at least remove the oem sub; the new sub could "push on" the oem one, making a rattle, and removing the oem sub will open up that hole to let a bit more sound from the trunk into the rest of the cabin.

I've also noticed that pulling the oem sub out made the fuel pumps noise noticeable, but that is only when the tank is very near empty. Add a couple gallons of gas and they quiet up. Unfortunately my xjr has been chained to my driveway for the few months ive had it, so I've only been able to throw in a couple gallons from a jerry can to start and warm up the engine every now and then. Also possible I just need new pumps.