I did not choose the premium sound system. Instead the original speakers (apart from the front center one) were replaced with Focal speakers (PS165FX, PC165). In the trunk we put a Rockford Fosgate sub (Slim punch 12") and a small amp (Mosconi 120.2, 350W) to drive it. To reduce the road noise the trunk and rear wheelwells were covered with the same bitumen mat Tesla has put small pathces here and there. All doors had the same treatment. Total cost EUR3000 (parts 1500, work 1500).
The effect is a much better sound quality, better than the premium sound system I would say. Noise level at 100 kmh and above was reduced by 2 db (S85, 19 inch wheels). Now my car is 1-1.5db quieter than my old Audi A7, which is a meaningful difference.
When you put in a sub, do use some time to get the level and phase right to get the bass you want.
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... forgot to mention that a level controller was installed below the dashboard. You can adjust the sub level from the drivers seat, better than diving into the trunk...