Re: how do headunits affect sound quality?
More expensive head units and cheap head units do not have the same features other than cd playback, power supply, am/fm radio, etc.
Beyond that is what makes the more expensive hu better.
Extras not found on "inexpensive"...cheap head units:
Better quality materials used to build the hu. Lasts longer, controls do not wear out nearly as fast, etc.
Maybe not a more powerful internal amplifier but generally one using better materials and construction methods giving a cleaner more powerful sound for the same wattage. For those that use the internal amp that is.
Higher quality materials also mean lower distortion and higher signal to noise ratios. Equates to more and better sound for the buck. Buy a $69 1000 watt amp at Wally World and compare it to a major brand amp of 1/4 the power and you will see what I mean.
Ability to expand: Cheap er inexpensive hu's either have one pre-out for a one or two channel amp and no more.
Additional inputs: As mentioned better hu's have inputs for usb drives, mp3 players and ipods, as well as playing mp3 and wma formatted songs. Cheap units don't.
Tunability: better hu's have far more adjustment features allowing for a much better tuned system to the car's environment...varied equalizers, time adjustments, etc.
Signal processing: again better hu's have much much better signal processing systems. Whether they be Burr Brown, etc. They produce much better quality signals which produce much better sound quality...this can be heightened or detracted by the speakers and/or amps used but better quality signal processing is a key factor in good sound.
There are many more factors but are enough to get the picture.
Asking about which brands have better performance is like asking which ice cream flavor is better. We all have our favorites and are VERY loyal to them. Or we have had bad experiences with a unit or two, and crappy support from the manufacturer, and will never have a good thing to say about them again.
Your best bet is to search around the forum for head unit reviews. Additionally go to the brands web sites and start reading and comparing specs. Also go to online stores such as Crutchfield, etc. and read up on the brands and models they sell. The better online stores also have reviews of the brands they sell by buyers. They are pretty good to get impressions on how people feel about the stuff they buy and use. Not to mention threads on this forum.
In a nut shell you always get what you pay for. You will never get a $500 quality head unit for $69 brand new. That does not mean you won't get a great hu for $200 that meets and exceeds your needs. That is the important thing you need to assess before you buy. What are your needs today and in the future...meaning next couple of years lol. That is the work you have to do in order to make a wise purchase maximizing the bang for your buck.
'04 Accord EX 4dr
HU: Alpine iDA-X001
Amp: Memphis 16-MCA3004
Speakers: JL Audio XR650-CSi with Memphis MC1-MT1S silk tweeters
2012 Honda CRV LX
HU: Alpine iLX-007
Amp: JL Audio XD400/4v2 4 channel
Speakers: JL Audio C5-650 Components
Sub: 10" JL Audio 10W1v3/sealed enclosure