Death Note is your favourite anime, yet you don't want to see the source materials used by Madhouse to create your favourite anime? You're a strange one, Mr. Waddup.
It isn't the worst choice you'll ever make in all honesty: The Death Note anime is one of the few examples of an anime surpassing the manga it was based on. It deserves to get 10/10 in the audio department, it's high budget (with lots of brilliantly used dramatic 'camera angles') and Madhouse actually made the last half of the story exciting by speeding through it. Still, a real fan would want to see what wasn't shown in the anime, and a huge amount of the second half of the manga was skipped.
With regards to Naruto, you should read the manga and then watch the anime; the best of both worlds. There isn't really much point when the Naruto anime was faithful to the manga, but lots of people like reading the manga first and then moving on to the anime.
And, on a Naruto related note, TBD raised their Naruto price by £3 or £4 a few weeks back. It makes me even happier that, thanks to the nasty knock one of the back corners of my set has, I ended up getting it for £42. It's just too bad I wasn't able to get a mint set for £60 -- No perfectionist wants damaged goods, not even if near enough 1/3 of the price gets refunded.