PO installed a little Kenwood powered sub under the drivers seat - it appears to be spliced into the speaker wires, and has its own little remote to adjust bass, but it’s a “set and forget” type thing.
Once it’s set right, you can just adjust the bass and volume on the OEM Stereo/Sync. If I’m not mistaken, most Sub amplifiers have a low pass filter, and at least some have an adjustment