Hey nymarty
I’m pretty sure you can only use one or the other. Why don’t you use your amp outputs to your sub. I think it blends better and you get some of your amps character. Just by some copper speaker wire from Amazon not expensive.
That’s how I connect my sub works very well. Just make sure on some amps like Pass labs you have to connect the negative to ground not the negative speaker output. If you don’t know ask your amp manufacturer if that’s ok.