[Question] Sound Quality Problem

Hello,
I'm not going to pretend I know a whole lot about audio set up or even dj'ing, but I'm having some issues I'd like to know more about and possibly how to fix them.

I bought a Numark Mixtrack Quad some time ago as a way to learn the basics of mixing while still being able to get double drops and such that would require more than 2 decks.

I'm using Traktor 2.6 and the sound quality I get out of my speakers is frankly horrible.

I'm using the Mixtrack Quad's integrated soundcard, and I only got shitty pc speakers atm. So my set up is basically an RCA to jack converter and my speakers plugged in there. I also have a decent pair of headphones (ATH M50) that I can't complain about as far as sound quality goes. I tried plugging my headphones directly into that RCA/jack converter to make sure it wasn't just my speakers sounding horrible, and basically every song clips terribly at medium volume.

I would need to set the master gain to -15 or so to get some decent quality out of it but then it's just too quiet. I also got the Traktor limiter set to a -6dB headroom.

Is there anything else to blame than the soundcard itself? Because I didn't find anyone complaining about it..

If anyone has any idea of what I could do to know the source of the problem or actually fix it, they would be the most welcome.