Yeah RCA really has no place in a PA -imho, with the possible exception of your controller master out (if it lacks XLR) to a mixer.
The way you are connecting it, it seems that your top end speakers get the full signal too. I.e. the part that your sub would normally produce now also get produced by your top ends, sort of defeating the purpose of the sub.
Ideally you would use a cross-over. Most of the time those are built-in to the subwoofer. Oftentimes in stereo.
So typically you’d go:
1) XLR (2x) to your single sub input L+R
2) Take the high pass filter outputs (L+R) and run those with XLR to your two active speakers, then hook up the passive speaker to the active speaker it should connect to.
I looked at the Rockville sub. It is labelled as “thru”, this would normally indicate that while it has a crossover to only allow the lows into the sub, it does not send the remaining signal (= minus the low) through to the top ends, but rather the entire signal. It might, but in that case it’s labelled incorrectly.
This is a sub-optimal way of running your PA, but I guess it will have to do.
I’d get a small PA mixer like that Yamaha MG06 if you feel you need some more control and want something to change your controller RCA outputs to a balanced XLR signal.
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