Things to look for when choosing the right head unit..
Usability: Your head unit is the part of the car stereo that you interact with most so it is vital to get one that "feels" good to you. Always look at a head unit in a store display and use it for awhile. Try to flip through radio stations and tracks on a CD to see if it is quick and easy. If you have issues with small buttons, imagine what it will be like when you are driving! Models fall in the same price range, so usability is key deciding factor.
Power: Even though many just said not to use the built-in power of a head unit, we know sometimes it is necessary. Bear in mind that the power specifications given by majority of manufacturers for head units are not accurate. They often use terms like "music power" or "peak power" which have little real meaning because there is no standard definition of those terms. If the power is quoted in "RMS" terms then it is usually valid. However, there is still one other place of misconception. Often manufacturers will quote power as "30watts x 4 RMS". The "RMS" seems to mean it is a true indication of power but they are implying that all 4 channels can produce 30watts rms AT THE SAME TIME. With a head unit, this is almost always not true. Because of the small power supplies in head units they can rarely output more than 15-60 watts TOTAL. This means that the power to each channel at maximum loading would only be 1/4 of that total. Some manufacturers are better than others about giving accurate specifications and a few models are available with sophisticated power supplies which have higher power output but they are VERY expensive. If you're paying less than 36000php for a head unit (and most of us are!) then your head unit will not have much power.
Pre-amp outputs: These are a definite for any serious head unit. These outputs enable you to run an amplifier directly without need for any conversion. This is the cleanest output of the head unit. Some units have multiple outputs and sometimes ones that are crossed over. Look for the amount and type that you need for your system but keep in mind future upgrades. One is sufficient but having two to four allow you fade, or adjust the levels of multiple amplifiers right from the head unit. Some head units now offer 4 volt outputs instead of the usual 1-2 volts. This can be very useful since cars have a lot of electrical noise in them. The 4 volt output is less prone to noise, however, you must be certain that the amplifier or crossover being connected to the output can handle 4 volts or you will not be able to use the extra voltage. If your head unit does not have pre-amp level (RCA type) outputs you can buy an adapter which will convert your speaker level outputs to line level.

Pre-Amp and output voltage
Pre amp is usually the control center. Your volume control, tone controls, EQ (if any) and source switching is usually at your pre-amp. The Pre-Amp is designed to be a very clean low signal voltage and take care of any signal modifications you might want to make to the signal (music) BEFORE the signal goes to the amplifier.
Pre-amp signal is usually LOW, it was designed to be low so there is less distortion at this stage. Home pre-amps usually operate with about 1 to 1.5 volts of output, the idea was to keep the voltage down as low as possible to keep the distortion down as low as possible in the pre-amp stage and then after the signal leaves the pre-amp we let the amplifier do what it does well.

In car audio we have used this very low level for years with pretty good track record. But in many cases in car systems there might be some noise. Some of the car stereo manufacturers have stepped up the output voltage of their head units in an attempt to aid us have a noise proof system.

Installers have always found that certain types of noises in a car system can be reduced by turning down the amplifier gain controls. But most of the time the customer gets his system fairly noise free but with no gain!! The system just may not play loud like it should if the gain controls are turned down too far. Turn the headunit all the way up and it still wont get loud. This is where the manufacturers come in with the higher level headunits! Now you can turn them higher, even when your gains are very low (and hopefully any bad noises are also very low)...

Some old fashioned find this added amplification (and distortion) on the headunits a turn off. But the majority of us cant really hear the added distortion and find it much better than the noise !!!

Some rookies find the added headunit voltage an advantage cause now they think their amp will play louder! NOT TRUE! The amp will still have its max output power and it will not change in most cases of the amps max drive can be achieved. Many rookies compare a 1or 2 volt headunit with a 4 to 8 volt headunit and they only notice that with the higher voltage the system gets loud at 10 (on the volume scale) and you used to have to turn it up to 30..!! Some rookies are fooling themselves into thinking this is GOOD! This is better!! But it may not be.

Ideally, a system has a good swing to the volume control... Too much signal voltage into the amp the system will get loud too soon on the volume swing. Not enough signal voltage to the amp and the system volume control can go all the way up and not get loud like you might like it. Setting the amp gain controls correctly will give you this proper swing!

But I still hear them say, "WOW, I got a 5 volt headunit!, it sure plays louder!, its worth the money!"
I just shake my head in amazement...

Head unit power!
Car Stereo head unit power is VERY deceiving! Head units all have HUGE power ratings on the Boxes or right on the front of the units! The highest head unit power I have ever actually tested was 13 watts (at a half way decent THD). So even if the head unit says 35Watts or 40 watts per channel on it don't EVER expect for it to REALLY BE more than 15! Just cant happen!

REAL amplifiers use LARGE power supplies in them with transformers to STEP UP the voltage So that large amounts of power can be made! Head units just don't have room in them For transformers or large power supplies!!!! So NEVER expect more than 15watts per Speaker out of head units! Some of the more reputable head units might LIE on the boxes but If you read the small print in the spec sheets in the owners manual they might admit to the 15watt figure!