My personal opinion on the upgraded sound system:
1) if you prefer to listen to mp3's, standard itunes tracks, internet radio and/or stream over bluetooth, the premium sound upgrade may not be worth the money. The sonic upgrades in the system focus (right now anyway) mostly on things you either can't consciously hear or may not care about so may not be worth paying for. Likewise, if "good car stereo" means you have to feel immersed in sound, it's worth checking out the standard system and playing with the fader. The upgraded system has really nice front-soundstage imaging which you'll mostly adjust out if you set the fader to give you that "immersed in sound" feel. So if you're going to do that, it's worth asking yourself if the upgrade is worth the money.
2) if you truly want high-end audiophile grade sound in your car so it almost always takes an aftermarket solution to make you happy, don't upgrade the sound. Put the $$ towards an aftermarket upgrade instead.
My personal experience (echoed by others I've read about) is that if you play CD-quality tracks (FLAC or WAV) and like imaging, a more open sound, detail and accuracy but not so much (in a car anyway) that you fall into category #2 above, the upgrade will make you happy.
To my mind, Tesla has made an interesting choice in where and how they've chosen to improve the sound in the upgrade. I think they tried to make the sound system deliver things that are much harder to do (or flat out impossible) in a noisier ICE car. Like reproduce detail that would literally get lost in the noise of an ICE.
I'm very happy with it. Yes, you can get better going aftermarket and spending a bit more money but someone would have to tear the car apart and I'd rather not go down that route.
If you do get the upgrade:
a) Turn Dolby off
b) Most earlier reviewers report better results with a boost to bass, smaller boost to treble and an even smaller midrange boost. One of those recommendations came from a recording engineer. Which is an interesting contrast to recommendations I've seen recently to drop the midrange and leave the rest flat. YMMV. Adjust to taste.
c) Adjust fader to taste. I leave it stock. But if you want to feel enveloped by sound, you're going to have to adjust the fade way to the rear. And you should read (1) above.
d) For best results, play FLAC or WAV using a USB thumb drive. IMO, the upgraded system seems to excel (for a car) at imaging, mid-high end detail and overall accuracy. All of those take a major hit when you stream over bluetooth or compress the music into an mp3 or AAC, even at 256/320 kbps. So if you do that, you're taking away a lot of what the sound system does best.
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I should add: future software upgrades are always the wildcard. What are they going to with Dolby, for example? Everyone who's seriously experimented with the system has agreed that Dolby should be turned off. Yet Tesla obviously thinks having Dolby available is a good thing. Are they right or is that just wishful (and expensive) thinking on their part? I imagine we'll find out one day. There's lot of stuff they could do with their sw/hw architecture. What will they do and when? My personal advice is buy based on what your ears hear today. But that advice is worth what you paid for it
.