Weird balance on the JBL system?

I noticed the sound was front heavy on mine also. I did not adjust the balance I adjusted the fader so the indicator line was on the front of the rear seat, doing that made it sound more even to me.

 

I have the same trim as you OP, and yes, I too witnessed the lack of surround sound when I turned up the volume.

For strength controls I used the Fader to put more strength to the back speakers giving you more of that surround sound you are talking about. I also put a dab more strength on the left side speakers using the Balance control cause I'm getting deaf and old, and the wife complains it's to loud. Works for me! I just fiddle with the controls till I get the sound I want, Congrats and enjoy your new ride!

 

same thing with the genII... i have to fade back and to the left because it doesn't not sound center to me.

it also tends to help to disconnect the front center speaker... some things sound better with it.. some worse... (depends how much static you get in the mids)

from what i've seen, the genIII is better yet still acts the same.

 

The JBL system on the Gen 3 Prius goes to a lot of trouble to create that effect; using a Digital Signal Processor (DSP), electronic crossover, and 8-channel amplifier.
The "ideal" sound it aims to create has a focus that's out front and center.....it's not intended to create a surround sound effect like some 5.1 channel systems are. The upper rear speakers are very close to the ears of the driver & front passenger; if they are too loud they will overpower the front speakers and the sound would arrive much sooner to the listener's ears....screwing up the sound that the JBL engineers were going for. I'm not a big fan of the way it sounds, but it does do a good job of achieving the effect it was designed to.

 

That's kinda what I was figuring you meant.