No matter how much I've spent on car hi-fi I'm never truly happy with it. OK so I haven't spent Pete money, but have Alpine X type front components, R type rear components, R type 12" sealed sub, V12 5 channel amp, 7.5" motorized screen / DVD head unit, 6 disc m/c and iPod converter.
Kits probably £2.5kish, but it never sounded too good in my old Leon. I started Dynamatting the car and it made a marginal imrovement as ambient noise was softened. Firstly having the tweeters firing in at you produces an odd soundstage. I've tried tuning it with Alpine's time alignment function but its still a bit artificial.
6 months on and I haven't even got round to fitting the Alpine speakers in my new Leon as can't get enthusiastic enough for the effort. Plus mounting aftermarket tweeters is a pain in the doors.
I did have 6x9's in my Lupo (MB Quart 3 ways) - have to say I didn't rate those either. Strange cuppy bass with little extension, and treble / midrange was just unpleasant.
As per Pete - spend all you can in the front. My components were £240 and I think they are still lacking