Hi
Just a thought re how you spend your cash.
The problem IMO with buying slightly better speakers\high end is that you will not get the best from them if you just run them from a head unit. The sound quality will be affected more by the output from the head unit rather than speaker capability. For example most head units that are 4 x 50w actually only put out about 18w RMS. So your 35w, 45w, or 50w speakers aren't getting enough clean power anyway let alone potential 70w+ speakers. The temptation is then to turn up the head unit beyond half way, but then you send distortion to the speakers. This distortion then makes it seem that the speakers are not capable of handling the power due to them sounding so bad. When in actual fact it's the signal being sent to them.
Take my setup for e.g. I have an Alpine head unit running the Alpine 45w speakers. At volume setting 17 it sounds ok, but it's not loud enough and the low end is very poor. But at volume 23 it's plenty loud enough but then the treble is harsh and the lower end just farts and rattles the doors.
However, when I send the "17" level signal to an amp, I can amplify this clean signal to a higher level. The result is a really nice loud clean crisp sound with no door farting (louder than volume 23 on the head unit). On top of this you then have the extra control of not sending anything below 80hz to the doors and letting the sub handle it. This makes the door speakers sound even punchy'er. Thus you have achieved nice sound with cheaper speakers.
So in other words, get the balance right on how you spend your money. A 4 channel amp can be had off eBay quite cheap and it will make a world of difference. Plus, you can upgrade speakers (or even your cheap amp) to your hearts content later, knowing that between your head unit and the amp you will be able to send enough clean power to make them loud and crisp.
I'd say that for now, don't go above your current speaker budget. Then save up and get an amp. I bought and installed mine for about £80. Sounds awesome. Give me a call and I can talk you through the install if you want.
