The number one component responsible for determining what your system sounds like is it's speakers.
All the electronics on the planet cannot correct for a poorly designed or installed speaker. There are several factors that determine what a speaker will sound like. There are many criteria evaluated during the design process of a loudspeaker, many of these are difficult to research as they are proprietary to each company.
Frequency response is what most people talk about. A speaker whose specifications claim that it will play music from 30Hz to 20kHz is not going to provide the tolerance over that range. Almost any speaker can do that, given a range of 30 or 40 decibels. In reality, ruler-flat response can only be achieved over a narrow frequency range. You can't expect a small tweeter to produce audible bass frequencies. Likewise, a big subwoofer is too heavy to respond to high ultra-fast high frequency signals.
Other factors that affect the performance of a speaker include phase response, radiation pattern and group delay. Phase response determines what sort of delay accompanies the reproduction of music emitted by that driver. Every speaker has some sort of delay in it's output relative to it's input. The drivers radiation pattern describes how it projects sound. Some speakers are very directional, or beamy. Some speakers exhibit very good off-axis response due to their design. These typically work well in the factory locations in cars, since very rarely do we get to set directly in front of a speaker.
There are certain misconceptions that run rampant in the field of car audio, and one of the most detrimental is that co-axial speakers don't, or can't sound good. People run down to Future Shop or Circuit City and grab a set of 5-1/4", 6-1/2" or 6"x9" Pioneer, Sony or Clarion co-axial speakers. Sure, technology has come a long way, but a good quality co-axial can outperform a poorly designed component set.
A woofer or midrange with the tweeter mounted centrally is capable of offering (all other things being equal) the best possible sound reproduction because it is a 'Point Source' system. That means, quite simply, that the music emanates from one point and thus, arrives at your ear at the same time. This is the trick to getting good sound in your car! There are good and bad co-axials. MB Quart and Focal offer some excellent co-axial drivers. And if you get the chance to compare a component set, to a co-axial driver using similar components (in a car), take the opportunity!
Up next, and equally annoying are the people who buy speakers based on their power ratings. Worse than that are salespeople that sell speakers on the same useless ratings. This is a waste of time! The wattage rating on those speakers is basically useless. That differs a great deal when it comes to woofers, but we are talking about midrange and high frequency drivers here.
I would be willing to be if you connected 150 Watt 6x9 to one channel of a high quality 150 Watt per channel stereo amp, you could destroy it quite quickly. The ratings are those speakers are usually the absolute peak power that the driver will handle before one of the voice coils becomes damaged. See the subwoofer page for more information on speaker power ratings.
Please do yourself a favour and buy speakers based on their sound quality and performance, not what they look like, or how big a power handling number is glued to the side.