Using RCA preamp line level rather than speaker wires for subs is best:
It is often better to remove the lows from the signal, send only the lows to the sub, and then your amp(s) for your mains will not need to try to make low bass and thus your main speakers will also be relieved of the job of producing lows.
That is how/what allows a low powered amp, often a tube amp, to be 'enough' with the help from a self-powered sub.
So, rca out to a crossover somewhere, often within the sub, the sub takes the lows and sends the rest of the signal back out to your amp(s).
IF you use both rca out from your preamp, or a splitter like shown, then your mono blocks and main speakers will still be trying to make all frequencies, including the lows intended for the sub. It's the lows that need the most power.
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