Aftermarket HU w/ Aftermarket Speakers

[Shunto]

That's a mighty heat sinked amp!

 

[Meccanoble]

Question: The PAC system I bought came with a SUB RCA I guess in preparation of an aftermarket HU install without sub outputs? Is there any way to connect OEM sub to this to allow control of subwoofer? Or is the PAC sub only function for an aftermarket sub which would be waste in this scenario because I rather connect RCA's directly to HU.