I have to disagree with this statement,
bad mix can not and should not be addressed at mastering stage,
simply if mix is bad, it should be re-mixed,
even if it's great mastering guy, 'cockup' made during mixing can't be re-done during mastering
although someone with great skills could try to minimize problems within the mix, but this is reanimation process, not mastering
Bad mix good mastering can fix, but I have seen cases where bad mixing/recording and the mastering engr couldnt work with it. Im with Tom all the way "IF YOUR SERIOUS BOUT YOUR MUSIC GET A PRO INVOLVED" best ignored advice on the planet.