It's true in the two you mention but in Jaguars for instance everything behind the speakers is Alpine and Mark Levinson in Lexus use Pioneer bits. That's what I mean there are to many variations to say one is good and one is not. All you can say with any certainty is that the upgrade will be better quality than the standard system.
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also
Just buy whatever car you want then send it to a good audio specialist to have upgraded speakers and amps put in.
The very best aftermarket systems will come in at a fraction of the price of new factory options.
EG a pair of high end component speakers is around £300.
A decent sub is around £200.
Amp to drive 4ch of speakers (4x 2 pair) approx £400
Single ch amp to drive sub £80
All can be hooked up to OEM head unit without issue.
If you want extra crispness then a DSP can be added but costs grow exponentially from here.
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A lot of modern headunits won't work with an aftermarket amplifier, especially if a fibre optic network like MOST is used to connect them together.