I'm with you. I have a 2020 TRD Pro with the JBL and I HATE it. I had a 2016 Avalon with the JBL system in that and it ROCKED. It was one of the best stereos I'd ever had, much less a factory unit. The JBL in the new Pro's is very Flat, almost zero bass, just sounds like a cheap radio to me. If the JBL in the Avalon was a 10, this is a 4 tops. You have to crank it way up before you'll notice any sound coming from the rear at all. It sounds like JUST the dash speakers are driving at you and they sound like cheap paper speakers.
My best friend has been a car stereo dealer for more than 30 years, I've got him looking for solutions and he hasn't come up with anything that will actually work with the JBL system because of the ohm ratings, etc. There doesn't seem to be a good way to take the signal from the JBL head unit, run it through an amp and drive some decent real speakers... I don't want to replace the entire head unit and spend $3500 on it either.
He told me if I take those crappy paper speakers out and put in something decent, it will be a 4 ohm speaker and at full volume, you'd only get about what you previously got at 1/2 volume... So for now, I just ignore it. Bigger fish to fry and all that...
Good luck. If you find a solution, I'd like to know what it is too!
The one suggestion he had was to get a bunch of sound deadening material and put it in the doors and rear hatch door. He figured cutting road noise might make the JBL sound better.
