I’ve never been one who’s able to leave “good enough” alone, especially when it comes to electronics. I like deep bass in my music. I outgrew my desire for twin 12-inch subwoofers decades ago, but I still like a deeper sound than what most factory stereo upgrades can give. I’ve been satisfied by various single 10-inch subwoofer systems for over 20 years. So it was a natural pursuit when I decided I wanted to upgrade my GTI. Of course, I “had” to add a ham radio and make the entire installation more complicated than necessary. I have shared a sophisticated 12V power distribution network on a separate page.
I had been mostly pleased with Volkswagen’s MIB II base stereo. It’s no powerhouse, but it’s more than adequate. I had originally planned to simply tap into the rear speaker wires to feed a signal converter, an amplifier, and then a subwoofer. I think I would have been satisfied with such a simple setup. Then VW decided to discontinue its first-gen Helix subwoofer accessory, meaning they slashed the pricing by more than 50% to clear their inventory. I had already purchased supplies for adding a subwoofer, so I already knew I wasn’t going to keep the Helix box after the initial installation and testing. But the super-clean wiring harness and 5-channel amp with DSP were too good to pass up. Helix is a great starting point for a system upgrade.
After installing the Helix system, I routed two pairs of the Helix amp’s subwoofer output into an Audio Control LC2i line output converter. The LC2i accepts up to 400 watts of speaker-level input, restores any bass that might be suppressed by the factory stereo, and then outputs line-level signals to my Rockford Fosgate Power T500X1BR 500-watt monoblock amplifier. I think Helix’s output is optimized for the six-inch subwoofers. The subwoofer output seems peaky around 80 Hz, which yields throaty-bass from a larger subwoofer, making it nearly impossible to achieve sub-bass without overwhelming mid-bass. I solved that challenge by adjusting the amplifier’s low-pass crossover frequency to ~60 Hz. Doing this blocks the 80-Hz peak while still amplifying about an octave of sub-bass. The factory door speakers are surprisingly effective at producing bass, even at frequencies down to 60 Hz. The subwoofer fills the bottom quite nicely.
I use a Rockford Fosgate Power T1S1-10 10-inch high performance shallow-mount subwoofer. My goal was to keep my spare tire and still have room for my other electronics. This subwoofer works in an enclosure as small as 0.4 cubic feet and with a mounting depth of just three inches. I use a PLC2 Remote Punch Level Control to fine-tune the subwoofer levels from the driver’s seat. It’s mounted just inches from the parking brake handle and is usually set to just 35%, leaving plenty of room to provide much more bass, if desired. In fact, 500 watts through this subwoofer is enough to produce ridiculous amounts of bass in a tiny car. I’ve shared a photo of the remote control at the bottom of this page.
The box itself is unique, mostly because it’s more than just a box. I wanted everything to be concealed beneath the removable floor in the trunk. “Everything” includes the subwoofer, power distribution and regulation, amplifiers, and a ham radio chassis. On top of that, I wanted it to be easily removable in case I ever decide to autocross or track the car. With that in mind, I included quick-connections for the electrical parts. The 3.25″ tall box has ~0.55 cubic feet remaining with the subwoofer mounted. I used Acousta-Stuf polyfill to fake more volume. The 68-lb panel can be lifted to reach the spare tire and whatever is left of the storage area; or it can be removed altogether. Getting the carpet applied evenly was my greatest challenge. It’s not perfect, but I think it turned out well. I will very likely build a replacement box and finish it with truck bed liner.
In short, I replaced the Helix subwoofer with a system that features double the driver size, triple the power, deeper bass response, and retention of the spare tire. Bonus: a 10-inch subwoofer in a hatchback provides even better bass than what I had previously enjoyed in sedans since there’s no “trunk” acting as a sound barrier. Those with the Fender system can duplicate my efforts using the parts and method I shared here. I have shared more photos of my project below and I have addressed the amateur radio system and a sophisticated 12V power distribution network on separate pages. Both are complex enough to warrant their own pages. Feel free to ask any questions about my rather unusual setup.