Z3 Audio Upgrade

Hi all,

Just completed an audio upgrade on my Z3, using the following :
- Hertz ESK 130 5.25" Splits
- JBL GTO-804EZ (2x80W, 1 x200W RMS)
- Ground Zero GZNW 6.5 (250W, 6.5")
- Pioneer DEH-5450SD

Going off the driver spec sheet, I built a sealed enclosure to fit between the seats using 12mm MDF, and managed 4L of volume which was the min spec for the driver. The enclosure was sealed with silicon, and I also added polyester filling to the box - about 50% by volume.

This is what the completed box looked like :
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Box mounted:
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For the splits, I first lined the existing speaker locations with a dynamat alternative, and then fitted the speaker. This was necessary as the 5.25" mid-bass driver was a little smaller than the OEM location.

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Once installed, the speaker seemed 'airtight':
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The tweeters mounted pretty easily to the OEM door tweeter pod. Didnt know that that OEM hifi system was 3-way - would have went 3 way if I did. Had to leave the 2nd OEM tweeter disconnected.

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Modified the OEM amplifier mounting bracket to hold the crossovers and the new amplifier :
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Mounted in the trunk :
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In terms of cabling, I did the following :
- 2x RCA pairs from head unit (for separate sub-woofer line in)
- New power and GND to amplifier
- new cable between amp and subwoofer

However, for everything else, I tapped into the existing harness. This is why the crossovers were located in the trunk - the OEM harness had speaker pairs running from trunk to each pair of the splits.

Now, Ive been tuning the system for the past two weeks, and while the there is a vast improvement over the OEM audio, Im not 100% satisfied.

Probably my biggest concern is that im not getting the lower frequencies. The driver is rated down to 45Hz, but I suspect the box is the problem in this scenario. Not sure if my expectations were a little too high...
To bring some perspective, the OEM system in the Mk6 GTI has more powerful lower frequencies than Im achieving here. Mid-bass however seems more powerful in this setup.

Anyone care to comment on the above ?
I suspect its the small driver combined with the small enclosure that is the cause - but wouldn't mind some advice here.

Will stick with it for a while, but will look into a passenger footwell mounted sub at some point in the future.

Thanks for looking.