Yes in my Defender. Focal speakers/sub/amp. Made a massive difference, but the standard 'system' is a joke as you might expect. Did headunit and speakers first, didn't make a huge difference as still tinny but a lot clearer. Once I added the Focal sub/amp it made a huge difference and sounds very good for a Defender!
That's where I'm at, the BMW standard is terrible.
I have also upgraded the head unit to an Android 10, again OEM is terrible. Looking to sort the speakers out now and will probably follow your approach regarding an amp at a future point.
Hopefully I'm able to do it myself
I think a lot of BMW owners also use Focal as most are the same size as they are in a Defender. Changing the speakers will give a lot more clarity, but for extra bass you'll need the sub. The Focal ibus 2.1 is excellent and should fit under a seat so no tacky crap in the boot etc.
Yes focal do an upgrade kit as do bavsound and rainbow components, the drivers are the same size as the oem and fit in the same door card/under seat brackets so nothing needs modifying. Something like this:
It depends how far you want to go with the upgrade but if you are decided on installing a separate amp then I would suggest you do it all in one go otherwise you'll be taking interior bits apart numerous times to run wires and cabling and the more you take it apart the more clips youll break! They're only cheap plastic clips and spares are easily sourced but its just more headache to do it in bits.