DJM 400 and JBL BOOMBOX DELAY

Hey guys, I‘ve been facing an issue trying to mix into my DJM 400 mixer and my JBL Boombox speaker. There is a delay between the headphone sound and the main output. I am pretty sure is something related to de the speaker processor, because when I connect another JBL little speaker into the headphones plug, there’s no delay anymore. Do you guys know anything I can do about it? Thanks a lot