Is it a Discovery 2? Is it an SD or SE? My SD doesn't have sub woofers but they are typically in the back door if you have them. It is a modular unit that comes with everything. Knowing LR it most likely will have the wiring harness located in the rear door panel for one harness fits all assembly line install during manufacturing.
The under dash harness should have the sub woofer plug as well. Power amp is under the drivers seat from what I understand. I don't know if there is an aftermarket stereo upgrade harness kit.