The branded audio systems available with many new vehicles these days have gotten better and better. This and other factors -- such as cars becoming more electronically complicated and automakers' audio systems becoming a better bargain in many cases -- has led to a decline in aftermarket audio sales.

But if you’ve already bought your car and want better sound, the aftermarket offers several ways to achieve it, without starting from scratch or even permanently altering your car. Several well-known name brands in aftermarket car audio recently introduced easy plug-and-play systems that use a vehicle’s existing radio and simply add more and better power and speakers “downstream” for dramatically improved sound.

JL Audio is a highly respected name in aftermarket car audio, and the brand has never been available as an automaker's premium audio option. At the 2012 Consumer Electronics Show in January, JL Audio introduced its StealthMod line of upgrade kits that provide add-on systems for three vehicles: 2007-and-up BMW X5 models ($1,899.95), 2010-and-up Chevy Camaro convertibles ($2,399.95) and 2010-and-up Kia Souls ($1,449.95).

Each StealthMod kit includes JL Audio speakers that drop into the stock locations, and a Stealthbox subwoofer enclosure that fits into existing nooks and crannies in a vehicle's hatch or trunk.  Each StealthMod kit also comes with a multichannel amplifier, adapter plates, wiring, a subwoofer-level remote and all necessary hardware for installation. If you don't own one of those cars, JL Audio says kits for other vehicles are on the way. And if you just want to add bass to your ride, JL Audio also has Stealthbox subwoofers that fit a wide range of vehicles.

Rockford Fosgate has long been a marquee name in high-performance car audio, and the company also provides premium systems to Mitsubishi and Nissan factories. When you buy a Juke SL, an  8-inch Rockford Fosgate subwoofer comes standard. If you already own a Juke, you can add Rockford Fosgate’s new ecoPunch factory-upgrade audio system. It includes a Rockford Fosgate amplifier and choice of subwoofers: dual 6-inch subs or a single 8- or 10-inch model. The system comes with a wiring harness to connect everything to the factory audio system, and the add-on components are designed to fit into the vehicle without alteration and work with existing factory speakers.

The 5-channel, 500-watt amp is from Rockford Fosgate’s Punch line and incorporates Rockford's signature Punch bass EQ to boost the bass. It also comes with a Punch Bass Controller for adjusting the level of the low frequencies.

Scion has always been known for providing owners with a plethora of factory-authorized customization options. Now the brand is offering options that take the sound system of 2012 Scion models to another level. OEM Audio Plus offers plug-and-play systems that can be installed in two hours at Scion dealers for the tC and xB. Systems for the iQ and hot new FR-S will be available soon, according to OEM Audio Plus. Each system includes speakers and an amplifier that replaces the car’s factory components as well as a subwoofer – all designed to fit into factory locations. Prices range from $999 to $1,599 and the systems are covered by a three-year/36,000-mi​le warranty when installed by an authorized Scion dealer.

I recently had a chance to hear the 10-speaker, 490-watt system in the Scion tC, and you’d be hard-pressed to get better sound at this price, and with less hassle.

Doug Newcomb has been covering car technology for more than 20 years for outlets ranging from Rolling Stone to Edmunds.com. In 2008, he published his first book, "Car Audio for Dummies" (Wiley). He lives and drives in Hood River, Ore., with his wife and two kids, who share his passion for cars and car technology, especially driving and listening to music.