Connecting a subwoofer to the 440 Pro dual

[cescrig]

I have a similar configuration, and use the RCA configured as a sub-woofer output.  I believe that's the recommended approach since it down-mixes the stereo signal to mono, which is generally what you want to drive a single sub.

Thanks for replying.
Let me explain: The RCA port configuration would be:
Preamplified output (mono)
Mix mono
Relative level 0
Filtering Full range
Volume control Variable
And on the subwoofer, I'll choose the crossover frequency and other parameters.
Is this correct?