Let's talk speakers

This is the sub box I made but I'm going to go with the JL active (powered) sub.


I had JL before and I've decided I want JL again.

Not cheap but I can now go to a 4 channel amp which will save me a little money of the amp. I have a 5 channel Eclipse for sale if anyone want's it, and I have the sub for sale too.

If I were to go with a sub, under seat, where do I start in figuring out what I need in order to complete this project?

I am rocking the OEM tape deck. I like how it looks as well as the sound of cassettes.