If you're driving an Audi A4 B5 (1994–2001) and want to upgrade your bass without giving up trunk space or tearing apart the interior, an underseat subwoofer is one of the smartest solutions. Compact, powerful, and easy to install, these active subs can breathe new life into your factory sound system. Here’s how to make the most of your setup.

Why Choose an Underseat Subwoofer?

Compact design – fits neatly under the driver or passenger seat.
• No trunk space sacrificed – perfect for daily drivers.
• Adds bass without needing a custom enclosure.
• Easy to power from your factory head unit or basic amp with high-level input.

The Audi A4 B5 Audio Layout

The A4 B5 came with a modest factory system. Front speakers are located in the lower front doors, tweeters sit next to the door handles, and rear speakers are in the doors or rear shelf (sedan only). If you’re replacing everything:

130mm components fit in the front doors – a great option is the Hertz DSK 130.3.
• 165mm coaxials like the Hertz DCX 165.3 work perfectly in the rear doors and rear shelf.
• For bass, the Caliber BC121US underseat subwoofer is compact and powerful.

Explore compatible speakers for your vehicle at Audi A4 B5 Lautsprecher.

Improving Bass Output from Your Underseat Sub

Here are tips to get the most from a unit like the Caliber BC121US:

Use the bass boost – this model allows +0 to +12 dB at 45 Hz, perfect for enhancing the low end.
• Adjust the phase switch – 0° or 180° can make a huge difference depending on the cabin acoustics.
• Proper mounting – secure the unit under the seat using supplied brackets to prevent rattling.
• Add sound deadening – especially in the front footwells and lower doors to tighten up low-frequency response.

The RCA Problem with Audi OEM Radios

One of the most common questions from Audi owners is: “Where do I plug in RCA cables?”

Short answer: you usually can't.

Factory radios in the Audi A4 B5 typically don’t include RCA (low-level) outputs on the back of the unit. That makes it impossible to connect a subwoofer or amp the usual way.

Instead, there are two options:

Use a line output converter to turn speaker-level signals into RCA.
• Choose gear with a high-level input, like the Caliber BC121US, which connects directly to the factory speaker lines.

More about OEM integration at Auto-Lautsprecher.

Using High-Level Input with a T-Harness Adapter

To make installation even cleaner, you can use a T-harness adapter:

This fits between your factory radio and the car’s main harness.
• One leg continues the factory signal to your door speakers.
• The second leg routes signal directly to the subwoofer’s high-level input.
• No wire cutting, no splicing – just plug, route, and tune.

Advantages:

Maintains full OEM functionality.
• Keeps the factory fade/balance controls.
• Easy to remove or reverse – great for leased vehicles.

Recommended Setup for Balanced Sound

For an ideal entry-level upgrade in your Audi A4 B5:

Front: Hertz DSK 130.3 component set (130mm, 93 dB sensitivity)
• Rear: Hertz DCX 165.3 coaxials (165mm, 93 dB sensitivity)
• Subwoofer: Caliber BC121US underseat active sub
• Optional Amp: A compact 4-channel amp like the Hertz DPower 4.300 if more control is needed

Want to explore more products? Visit Autoboxen.

Final Thoughts

An underseat subwoofer is a fantastic, stealthy way to add low-end punch to your Audi A4 B5. Combine it with efficient front and rear speakers, and you’ve got a dramatically better system that fits factory locations and works even with the OEM radio.

With smart installation and tuning, you’ll enjoy a much fuller soundstage – and you won’t miss having a trunk-filling sub at all. Best of all, it’s an upgrade any DIYer can do with a bit of patience and the right parts.

Happy tuning!