If you’re that worried about sound quality, you have to consider that the Prime players have some processing faults right now that high-end digital mixers reveal. Practically anything else with a digital out and keylock off into the MP2015 would sound better than the complete Prime system, and a CDJ/XDJ with a digital out and master tempo off into the Rane would be near reference sound quality (only non-reference aspect I can think of being the Rane channel isolators always in the path). Hopefully not just the x1800 processing, but especially the players’ own is further improved such that top digital mixers really are worth it on them.
You’ve got the Prime players, so you like their features. If you’re hard up for cash that this is such a tough decision, you might want to get a cheap, used analog Ecler or Xone four channel. A Nuo 4 or Xone 62 will sound nice with the Prime players… more forgiving of the players’ own processing faults right now, IMO, too. The 62 would have filters, and you can even use it as a rotary if you put the headphone in Aux mode, mix with the aux pots at the top, and use the aux out on the back for your mains. You can do some funky stuff with the pan, crossfader assign, and filters that way, too.
Later you can save up more money for something else… like even an X1800, and by then it might have better firmware. A used, cheap 62 if you take care of it will retain its value.
If you can afford the X1800 comfortably at some price you can get it for right now, really only you can say whether it sounds good enough already for you with the players. Honestly, I’d be curious what your opinion would be on it if you got the new Denon… not trying to pressure you or anything.