Best Sounding Adapter - Making RCA into Banana or Spade? Thanks!

You could get a line out converter if you want to take speaker level signal; Or a line out attenuator, fixed or variable, if you want to take signal from line level. Try Amazon or Crutchfield.

Why not just just use  RCAv Y connectors from the integrated to the sub?

In my case, I was doing that from the beginning. However, it turns out the amp pumps too much power into my sub. You can tell right away because it leaves no room for volume adjustment on the sub, and it doesn't sound right—it just rumbles. That's when you need the aforementioned attenuator.

The use of Line out Converters (LOCs) is well established in autosound, as factory head units lack preamp outputs. I use a Stinger LOC in one of my systems to hook up a pair of subs that lack speaker level inputs. While other options may exist, this is surely one.  Why doesn’t OP say exactly what they are working with and what they are trying to do with it?