Do I need a sub?

First sub in 20 years.
first was a 12” cerwin vega front firing 
long times ago.

  Hoping this will add some low end.

 Even. My sanders sound pre does not have a sub iinput,
or my old onkyo p308.  
Such a shame, I. Have to do the wiring exchange for the sub.

 I very much like the rca automatic sub thing, but my gear won’t allow it.
and no I’m not changing, my stereo sounds amazing, and took me 35 years to get where I’m at these days.

Hooking up a powered sub.

 All my preamps are older w no dedicated rca to sub.

 Dislike the wiring amp to sub, then speakers from sub to amp.

is there a better way?

help

Yes.This is what I use:https://www.audioadvisor.com/prodinfo.asp?number=AQM22FHRD
They are very well made and snug fitting.

This works how?

two rca into sub, and a single rca into the preamp white pre out??

Amp output red and white both into the audioadvosir connector
and into the single rca of the sub?