Did you try to run one of the distorted ones through MP3 gain? Maybe the signal is just too hot?
Did you try running the distorted ones through just a mixer and an amp, in other words leave the 3000 out of the mix and see if that helps.
Are all your mp3s acting that way or just some?
I would see what the gain is on the ones that aren't distorted as compared to the gain on the ones that are.
MP3 gain will tell you if there is clipping present and let you remove it.
Select the option don't clip when doing track gain.
I'm not an mp3 gain "fanatic" and, of course, it won't solve all problems, but I'd check that first.

I did a job last night and there was no distortion at all on none of the mp3's. The only thing I did different was I didn't use my laptop output booster. I have this little output booster that I bought at Radio Shack that you plug into the output of the laptop, which happens to be the headphone output, that gives a little boost to the output of the computer.
On my next gig I'll try using it again and if I get distortion I'll know that that's the problem.
I'll keep you posted.

Thanks.