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

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?

I'm not a subwoofer guy. Is there a sub with RCA inputs on the market that is designed to be connected to speaker output connections (that's a lot of current for an RCA type)? I would recommend that you send an email to the sub maker include all the information on your amplifier, make, model, power output, speaker impedance, type of terminations, etc. There is a possibility, and I hesitate to mention it for liability reasons ;) but is your sub a powered unit specifically designed to use the tape monitor outputs on your amp, or a line-level output? Perhaps if you give the forum information on make and model of sub and amp, someone here has saved the instruction book, or go to hifi engine for user and service manuals. The amount of current in a line-level, or RCA circuit compared to the current from a speaker output from an amplifier, is like the difference between a yard sprinkler and Niagara Falls. Niagara Falls, very dangerous, lawn sprinkle safe for kids and dogs. Please contact the sub maker or the place of purchase.

To do this, you need a "preamp out" section on the back of your amp.  Otherwise it is a no go.