It’s a standard upgrade solution. If the music sounds inadequate, upgrading speakers and their fitting/surroundings will improve to some extent and adding a multi-channel amplifier with a DSP again feeds more power and better control to each speaker. The ideal (without going mad) is to do both, upgrade speakers to something better but not perfect and feed them with a slightly more powerful amplifier with the benefits of a full digital sound processor. There’s a bunch of technical stuff to look at, crossover points, phase, balance and then equalisation but if you have a good shop, they’ll set up the whole system and all you need to do is play your music and change the volume.
I know a few guys who do a lot of this type of install into newish cars that could have been upgraded but, for the price, they were significantly better in terms of improving the quality. They are also the guys who occasionally pull out the whole interior and build a new dash, boot and doors/a-pillars with installs that cost more than the car was ever worth.
Most would upgrade all front speakers plus whatever sub was installed and change out the amplifier whilst keeping locations as the manufacturer intended and the whole car looking absolutely standard. The difference that would create would suite most, even aging audiophiles.😂
I would suggest this is only worth doing if you have the capability to undo it yourself (or pay the installer) and maybe put it into the new car if you’re simply changing to a newer version of the same car.
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I'll try to give my honest opinion but am always open to learning.
Mark