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.