REL high Level + Class D amp help pls !

hey guys i really need help !
so i own 3 REL subwoofers right now and i ordered this class D Amp from apollon audio (purifi) 5ch

 

REL is saying that its not a problem to connect high level with class d amps and that all you have to do is simply not connect the black wire to the negative (black ) binding post
however they mention and i understand that doing so may / likely result in humming noise/sound due to lack of grounding? (which im not sure what it means)

Rel suggest if that happens to then connect LFE into unused RCA /into sub in/out?
does that really solve the problem? because i see some reports in forums that it doesnt . .i even saw someone mention something like "high level digital cable" which agian i dont know what it is.. some others speak about making the black wire have RCA connection and connect to unused RCA? again will it solve?

and also i see some mentions about connecting both LFE and low level instead of high level? but what is this low level? how it different from the high level? and i saw it connect to RCA how does it differ from LFE? also my preamp(avr runing preamp mode) only have 2 subwoffer input ? what do i do then? how can i even connect 2 subs with both LFE +low level? do i use RCA splitters stuff? do they even help? do low level take signal from speakers? please explain to me and help me find solution and way to connect my subs with the new amp ive ordered ! thanks alot !