The sound quality is very poor with the factory stereo in the VZ. No treble response. Has anyone just upgraded the door speakers and not the head unit with good results?
Hmm.. mine had a factory 6-speaker system (4 doors, plus two dash-mounted tweeters) and the treble and sound staging was quite good for a budget stock system, it was the bass handling which really let it down (and it distorted easily if turned up loud).
What brand speakers did you settle on? Did they fit in without modication?
If you get 6" speakers they'll fit without any modification (except you'll need to remove the factory speaker plug and solder/crimp your new ones on)
6.5" may need some modification of the pod to fit.
When it comes to brand, you really need to choose for yourself because everyone has different listening preferences. If you walk into JB HiFi (or any car audio store) and listen to a few they have up on the board you should be able to find one you like - some will have more bass, some less.
Rule of thumb with JB and their cheaper speakers, is when it comes to talking price aim for about half of the sticker price. They put huge markups on these. Several times I've bought two pairs for the price of one (becomes less true of the higher end speakers, but there is still room to haggle).
Upgrading just the speakers will give you much better sound, but if you want
really good sound you'll need to either put an aftermarket head unit in, or do what many of us (myself included) have done, and get a modification to your stock head unit so it's outputs a signal which can be used to run an amplifier and some high-end speakers. There is a bloke on these forums called Phreddy who makes such a mod (
https://forums.justcommodores.com.au ) should you want to take this path.