That would be why. Component speakers are made to allow the high frequencies of sound through the tweeters, and the mid sounds to the door cards. Head unit wouldn't be able to provide the power to them to begin with as most head units are around 45w RMS (so about 10/15w per channel). Component speakers usually need at least 60w odd each!
I have a pair of Infinity components in the front and my sub handles the bass, all of which are powered by that JBL amp I mentioned earlier. That set up is all you need if you just want some good decent quality sounding music. Had the set up for almost 4 years now.
If I get all the kit how much would it cost to get someone to fit it all for me? All the amp wiring is already set up
You would probably need to get the wiring checked and possibly redone to make it safe with correct fusing etc.
I think Jbl Amps are good for the money. Depends what setup you want. In my lupo i have a single rockford fosgate amp running a crystal sub and Rainbow components. In the mk3 i have a two jbl amps. A 600wrms amp for the sub and another 100wrms per channel running some rubbish(IMO) focal comps.
Whilst the Golf is louder, the lupo has much better quality sound and tighter bass.
The Components are the most important part, so spend as much as you can on some decent ones.