Why can't I get clear sound?

I have a simple setup just to get me started in high-quality audio, which consists of a NAD C350 and Axiom M22ti's. The interconnects I'm using are cheap monster cables that I had lying around.

Now my problem is that I can't get clear sound from my audio sources. I have the speakers setup on my computer desk and I use the computer's output to play mp3's into the amp. I have many high quality rips from CDs but they sound muffled, playing a CD in the CD-Rom drive sounds muffled, downloaded songs sound muffled along with everythig else. I have even plugged in a portable CD player into the amp, my iPod and my Rio Car deck but they all produce a similar muffled and muddy sound that just doesn't sound right at all.

The only way I have been able to lessen this problem is by using a plugin for Winamp called the "Wow! Thing". When I turn this plugin on it sounds like a blanket has been lifted off of the speakers because everything sounds clear, the highs are more striking, and the bass is actually present!

I have a feeling that maybe all of the sound sources I have used are crap compared to any real good CD players. I have a better CD player, a Sony 5-disc changer, but I haven't had a change to hook that up yet, and I don't even know if it'll be any better than a portable cd player.

I just wish I could get sound the way it was meant to be heard out of my amp and speakers. For me, being still a kid and in school, the $700 I spent for the amp and speakers is quite a bit, but I'm willing to do a lot more if I can just get clear sound without having to use stupid winamp plugins and equalizers.