Rca → xlr
So I just got the full pioneer 350 setup (applause) haha, but anyway - it only has RCA out ( 
common pioneer).
Does RCA → XLR cables deteriorate the quality?
no.
Buy a decent cable.
I got two Live-Wire’s that are rca to XLR from Guitar Center and coupled them with XLR cables to go to each speaker and so far it sounds good 
BTW is there any benefit of routing it through a sound mixer (like lets say a small behringer or mackie mixer? )
Actually, technically, it does “degrade the quality” but only in that the amp or speaker that takes XLR is a balanced input but the output of your mixer is unbalanced. It’ll only effect the signal by not having the common mode rejection that a balanced signal will have(rejecting noise/interference) and overall signal level. Balanced signals are generally +4dbu and rca connections are generally -10dbv. You shouldn’t worry too much about it though, you’d only want to keep it balanced if your mixer had balanced outputs.
If you have the $ to throw at one, get this:
Yeah, I wouldv’e suggested a matchbox like the ones above but you really only need them if the run from you mixer to your amp or powered speakers is long, more than say 6-10 feet or so or if you’re experiencing static or interference.