I don't think the standard fit audio has an external amplifier so you'll need to add one yourself - whick means more work and expense.
You can change the speakers to higher quality units without adding an amplifier - but that won't be enough to upgrade the bass to the level of the professional system because it has two subwoofers beneath the front seats. Of course, volume levels will not be as high either.
I'm afraid you'll have to take it to a place that will have to do extensive work. I've done it before with past cars and the sound quality will easily surpass that of the professional audio system - but at a cost. Some of these companies are used to upgrading the audio in expensive cars like Bentley's.
One such company I've used in the past is Prestige Audio:
http://www.prestige-av.co.uk/CarAudio.asp
They can work to a budget so it's worth giving it a try.