Slight power boost, and a bit of hum boost too
As a part of my car stereo upgrade, I first went on to change the stock speakers to get a little boost to the low-ends (The mid and highs were good enough) The Polks I ordered made no difference to sound quality, so I went on and got the Alpine KTP-445U. After installation, I did not notice any boost to the lows. I then went on and replaced the Polks with the OEM speakers and that made all the difference. Contrary to what I expected, the OEM's despite their smaller magnets delivered a better punch with the Alpine power pack than with the high(er) end Polks. Overall, I am a (tad bit) more satisfied with the sound quality of my car's stereo with the Alpine KTP-445U installed. So why the 3 stars: The Alpine KTP-445U is an amplifier, and like (most) amplifiers, it will also increase noise. Not quite what I expected from an Alpine unit, but it is what it is. At low volumes, bass output is better as compared to the stock setup but at very low volumes the noise can be heard a bit, if you are my types with sharp (dog-like) ears. I got the unit installed by a pro, paid about a 100 bucks for the installation, but they did a good job at setting the gain. Check the Crutchfield Subwoofer Tuning article for some references you can use to set gain at the correct level Nevertheless, if you are looking to boost your existing setup without a sub, then the Alpine KTP-445U sure is a good way forward.
Pros: 1. Tiny little invisible unit, fits inside the dash 2. Slight boost your existing set-up, decent boost to the low-end frequencies
Cons: 1. Expect a slight hum from your speakers at low volumes, which is not noticeable at higher volumes