Is this the right way to use this crossover?

If you are looking for hooking your subs up from a RCA converter box from the back speakers a crossover will be pointless, the bass from your back speakers will just send the bass to the converter box and to the amp via RCA and you control your bass through the head unit and you also can make your subs louder by putting more sound to the back speakers lol. I did this and your subs are loud from this due to the higher power leavinging the back speakers through the box. The downside to doing this your subs don't sound as clean but its not really that noticeable. But just to get back the question i think that crossover will be pointless to have if you have that rca converter from the back speakers. :)