Budget Upgrade

Since there seems to be many knowledgeable people here I’ll ask some questions.
I have an '09 Ford F250 with the better of the factory stereo systems. Can post a pic of my dash later if need be. My system ties in with Sync, stereo/dvd system, navigation, and bluetooth. I’m not looking to replace the whole system.
Is it possible to just add an amp to what I have to get some more performance from what I have? Back in the day we spliced in booster/equalizers to what we had. Loved my Pioneer Supertuner! I might still have a booster/equalizer in a box in my attic…
Sometime down the road further upgrades might be in the budget. I have an extended cab and there is a box under the backseat with a sub in it. I originally couldn’t see the speaker and thought maybe it was something to do with the jukebox feature that I have which allows you to load CDs and store them. Maybe I could look into a new speaker box with speakers. Any input would be great.