A typical 6.5" driver (rear door) is not a full range driver unless it is coaxial. We have a mid-woofer in the rear doors, in fact I have one sitting on my desk right now and "midwoofer" is printed on the back sticker.
The UHFS system doesn't use full-range speakers. It has midbass/midwoofers in the doors, mids in the dash and hatch, tweeters in the A pillars, and a sub in the hatch floor. Each speaker has it's own range and they all compliment each other. That's how a typical 2,3,4-way system works. Either using passive crossovers (physical) or active crossovers (digital, done with DSP). Tesla uses DSP built into the MCU. The rear doors are going to play (some) bass, low midrange, and midrange. High midrange and above will be the mids in the dash. This was already verified by another user with a test disc.
I'm getting a full system installed in my S right now, which has front 3-way active, but I'm also adding a center channel and 4 rear speakers. The center and rear speakers will play very specific frequencies to compliment the front stage, but they will not sound good on their own. If you were to play music out of only the center and rears it would sound really really bad. The "fader" if implemented properly should move the sound stage or perceived center, not just the bare signal to the rear drivers.
Again, I'm not saying Tesla didn't break or change something, since I haven't hear before/after of any of your systems. There very well could be issues or changes and those should be brought up to Tesla and hopefully fixed by them. I'm just trying to set expectations for how a fully active sound system should behave. There is a reason why you don't hear sub-bass or mid/highs from the rear doors.