Well, it initially was a premium system with enough power to actually cause body rattles and speaker damage if abused. Some customers chose to abuse it and then claim warranty repairs.It's a shame that it's marketed as a premium audio system when it's not.
If you are getting usable sound out of the second row doors then Toyota made another change. In my 16 Rav the bass comes almost exclusively from the front door speakers. However, you'll never know with the dash speakers turned up so high. This is one of my first attack vectors in this post:but in my 2018 RAV4 Platinum there's not near enough volume, the JBL sub speaker in the rear cargo area is damn near silent, all the sub frequencies are in the second row doors and most of the sound comes out of the dash.
The issue with any upgrades is that significant vehicle functionality and diagnostic capability will be lost by replacing the head unit. Now the rear JBL amp has fiberoptic inputs which might be a standard protocol like SPDIF or Toslink which may provide a way to access the HU signals. It now appears that Toyota may have just mucked up the firmware in the DSP amp and not the HU as I previously thought.I suppose I am just going to have to upgrade it at some point. I don't recall what the cost was for the premium system but it probably was a hell of a lot more than putting in your own premium system.