I had this same issue with my Prime 4.
At work we have a DJM-850 and I used to plug the Prime 4 into this via the RCA master out and it sounded great. Then I decided to remove the DJM-850 from the equation and I used the XLR output where the DJM-850 once stood.
It sounded similar to how you described. Lifeless and muddy, almost as if the phasing was out on the connections. Did you connect it this way? Have you tried the RCA master output? If the Prime 4 had never been plugged in via RCA then I wouldn’t have noticed but I could instantly tell and this is coming from someone who used it seven nights a week on that system.
My diagnosis was that there was something funky with the XLR balanced master out which seemed to leave the sound muddy and hollow, yet RCA was fine. I just rolled with the RCA afterwards.