Stereo upgrade

I’d like to upgrade the stereo, and I’m looking for some advice as to what to upgrade exactly.

I seem to have the S676 HiFi speaker system, combined with a Bluetooth-module for phone calls only (no streaming) behind the back left panel in the boot.

There seems to be an Aux already programmed into the CCC, but there is no Aux-input to be found. I’ve checked the dash, glovebox and A/C controls for the backseat.

I have speakers and tweeters in the front doors, speakers in the shelf behind the rear seats (no speakers in the back doors) and a center speaker in the front, just behind the iDrive screen.

Short term I’m happy with an aux and some new speakers in the front, as I’m missing bass and depth in the sound.

Is that something speakers will fix, or is the amp just that bad and would I need to do more?