For starters, NO, your Mackie isn't crap! Its one of the toughest, cleanest mixers I've ever used! I swear by mine after the blizzard of noise even mid-line mixers from other companies have put me through. Anything that will deliver cleanly while in use under a mass of radio towers...!
Second, while I'm hard-pressed to give you a definitive answer on the FX/balance mismatch, it IS the case that headphone jacks have a different impedance because they're intended to drive them little cans instead of outboard speakers. This no doubt play a part in your problem. Sometimes that happens to me & sometimes it DOESN'T. Delicate mixes lose more than pumpin' pieces, so it's possibly a matter of sufficient gain.
I'd say e-mail Korg.com AND ask a local tech /techs about that. There are so many odd fringe behaviors in e-music, even between models from the same company, that you have to just ask around. Even Craig Anderton doesn't know it ALL!
There's also no law that says you can't try driving things directly FROM the headphone jack. Juggle your settings a bit and see how it turns out. I've had this method yield a good sound, especially low-level things that sounded a bit fuzzy to begin with. If you're going techno, it might even add some character you'll like. Adapter plugs uber alles part of the time, y'know! If you get more info, please post it.