A prefab sub box is basically an enclosure for your speaker that is already made off the shelf. You don't have to measure it or make the effort to build it, you simply plonk your speaker in there and connect it to your car amplifier. Now in terms of the best prefab sub box out there, they are all the same interms of build quality. They are made by car audio websites to make the most profit from them. They don't really care about build quality and the sound you are going to get. They are just made for people that don't know there car audio stuff to purchase. We have even seen some websites that sell prefab sub boxes that don't even tell you the specs out it. But if your on a budget or new to car audio and you have purchased a prefab sub box you can transform it into a better product, so don't worry too much.
We had a customers Alpine type r 15 inch first generation subwoofer fitted into a prefab box. Pushing around 700 rms the box started to self destruct apart. It was wobbling, the two pieces of wood were moving apart and lots of air leaks you could feel around in the corners outside the sub box. Overall, something that we wanted to replace with a custom subwoofer enclosure because prefab vs custom sub box is an easy choice to make if you have the right experience and tools readily available in the shop. If for some reason you can build a custom subwoofer box right now, you can atleast make your cheap prefab box perform better. We will explain how you can today.
Bracing. This will make the prefab subwoofer box sound better and louder because the box structure will be stable. This can be done by placing two strong wooden dowels in the left and right sides inside the enclosure. Applying them with glue and screwing them down from the top of the box. This will stop box flex that are apparent at low bass frequencies that are deep that move lots of air.
Sealing up air leaks. Prefab boxes are always known for leaks. This is because the builder doesn't want to waste glue as their profit from the box will become far less. I know its bad, so get hold of some wood glue and go around each and every corner, including sides to ensure an air tight seal. Also go around the speaker terminal cup as this area is prone to leak air out.
Use bigger gauge speaker wire. We have seen a 1000 watt rms subwoofer installed with the stock 18 gauge speaker cable supplied by the sub box builder. They do this because the less copper inside the wire and more plastic insulation, the cheap the speaker wire is but looks good on the outside. Replace it will nice thick 12 gauge speaker wire for better power handling and a cleaner effortless signal.
Polyfill. This can be used to make your subwoofer sound more deeper and louder because it makes the sub think its placed inside a bigger box. The great thing about this is you can find it in pillows and extract it out and place it inside the box. There is no correct amount to add because it can vary so play around with different amounts. If its a sealed box then you can just stuff it in, you don't have to use carpet spray glue to keep it stuck on inside the walls like with a vented / ported design.
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