The helix is only a tiny subwoofer. That goes in the trunk like the stock Dynaudio subwoofer. It has like two 6" drivers and is not that great but enough to fill in the low end/bass. I doubt Nabooda fits it, they dont bother with anything....at all! Even if you offer to pay.
Its not that powerful, but it could suit your needs as the Gti is a hatch and the bass has enough room to travel due the trunk being open to the car. The stock speakers are fairly decent....and a little bass wont hurt. On a jetta or something that would not be enough as the sub would be in the trunk that is separate. It comes down to personal preference. For me, yes the intergration and look was nice, but for the price you can get much much much better. So depends on what you want.
On the other hand, I had a subwoofer installed in my Gti, and that was enough to improve the system for the time being. I had a 12" rockford 300W enclosed active subwoofer. It was actually a little too powerful and on a scale of 1 - 10 I would keep the bass at 3.5, and that was more than enough LOL
http://www.golfmk7.com/forums/showthread.php?t=3396 <-- my post
This allowed me to keep the speakers at 0 bass (right in the middle) so that a little bass goes to the speakers too and creates a surround effect.
If I had not sold my car I would have upgraded the speakers too and used my sub to its full potential.
Check out gulf wireless in al quoz. They are right after VW showroom(towards Dubai) - pass nissan and mitsubishi and then its in the building with exotic cars and 3m tinting. Their other name is Car Audio People (its a building that is curved so the road curves with it). You won't believe it but they were recommended to me by a VW sales guy, who has since left Nabooda.
Anyways...They had a subwoofer that I wanted, it was flat, 12 inch, 300 W power and by Hertz which makes amazing speakers.
http://www.hertzaudiovideo.com/car-audio-subwoofer_box-energy-ebxf25_5/
Don't be fooled by their shop and how shit it looks. They do top notch work!
I only found out how good they are after getting my sub installed at Yellowhat and I did regret it, not that Yellowhat was bad. They are just a rip off.
Gulf wireless can do a full system for AED 5000 which is what I would have done if I could turn back time. I even had the full system mapped out. Hertz Energy 6.5" components front and back. 5 channel amp and Sub. Or a little more expensive but a 4 channel amp for speakers and a mono amp for the sub. The sub alone + amp and install was 2000....Same as I paid at yellowhat for a sub that not need an amp....see what I mean by rip off lol - Also yellowhat charges a butt load for installation (150 per door and 300 for sub in addition to all cables and stuff). Gulf wireless will give you a flat fee of 500 for the full car. And they do bargain. I am not affiliated with them, I just did my research and I am telling you exactly what my plans were
A friend of mine with a Gti as well has installed the Hertz Hi-Energy series of speakers from gulf wireless and he said they are bloody brilliant. The last time we spoke he had already installed the speakers and paid close to 4500 or 5000, without a Subwoofer..He was going to install the sub at a later stage (dont know which sub) but I have not spoken to him about it since... But...they are ofcourse more expensive. Energy series would be more than enough in my honest opinion.
Adding a subwoofer did not affect my warranty and I even had a flat battery once and they replaced it under warranty even with the sub installed.
Either way, my recommendation is the 12" flat subwoofer from Hertz as it does not take up much space at all. If I could go back to the day I installed my sub, I would stop myself and get the hertz.
All the best! (Sorry for the long post, I like to cover all points in one go

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Edit: Someone I know from the vdubz UAE group got his FULL CAR done at fos concepts so check that out too if you like, they can even do a custom install just like Gulf Wireless (i.e. build an integrated sub etc.).
I just know gulf wireless and have seen their work first hand.