I don’t have my unit in front of me to compare, but something sounds a little off in your settings. I also don’t use Traktor, so I’m just guessing a little on some of those settings. But it sounds to me like you have your software set to external mixing, and the Denon set to PC mixing, so nothing is actually acting as the mixer. If you are trying to use the onboard Denon mixer, leave your software settings the same, switch the Denon to ‘Int Mix’, switch Ch. 1 and Ch. 4 off midi to something else, switch Ch. 2 and Ch. 3 to USB and see if that helps.
I’m cleaning up the studio a little this afternoon, so I’ll try and unpack my 6000 and take a closer look if nobody else has offered advice by then…