The problem is the the 510 only has 20w of power, the stock speakers are actually quite good
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THIS.
Amp first.
The biggest improvement will come from adding an amplifier so that the speakers are getting a clean signal at higher volume. Try it and I bet you'd be surprised how well standard golf speakers can perform.
My audio installer rates the mk4 golf and mk5 golf speakers highly, better in his opinion than the Bose that were in my other car.
Also consider disconnecting the rear speakers altogether. You don't need them. When you listen to a hifi or go to a concert, you have the music soundstage in front of you. The rear speakers are not needed in a small car.
In my mk4 golf, I ran a pioneer HU, OEM front speakers, disconnected rears, a 12" JL W6 in a custom box, all powered by a pair of genesis amps...sounded great.
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