Help with issues between Controller and Mixer - sound coming out filtered

Hey all! Hoping someone might be able to shed some light on an issue I had recently. Was playing at a Gym. Set up: Serato, Controller (Pioneer DDJ-SB2) into Mixer (Numark M6 USB) - use the controller and laptop all the time but mixer was venues. The sound coming out sounded filter and not clear - the bass was varying a lot and had to be low on both mixer and controller but just couldn't get it to sound clear or right. Anyone had this issue before or have any advice? Supposed to be playing again next week - thanks!

Have you checked the cables you were using to connect into the Numark mixer, tested on some other equipment? They are under 2m long?
Also, are you sure you didn't plug by mistake into phono input on the mixer?