ICE - in car entertainment
HU - head unit
SPL - sound pressure level (concentrating only to achieve excessive loudness  )
SQ - sound Quality (concentrating only the front component. and of course a sub to help a lil bit on the front)

EQ - equalizer

correct me if wrong

And also the fact that the subwoofer is actually needed to fill up the last octave of the music spectrum, well, at least, in most of the songs i listen, even crossing my front components at 63Hz doesn't help, definitely need the sub to fill up the rest of the low end for a fuller music experience. Most front components wouldn't be enough to cover the subwoofer part most of the time, even if you are running a 3-way with an 8" subwoofer(which, most, doesn't really have much sub-bass, except, i think, the JL 8W7 and i doubt u can fit the 8W7 in the doors anyway) in the doors

Yes, output level/impedance does make a difference. Output level higher meaning you can set your gains lower at the amp, thus reducing the stress on the amplifier, though, it doesn't necessarily means better SQ from it. Just that with a lower gain, it'll usually reduce floor noise.

As for output impedance, the lower, the better as the signal have to go thru less resistance

Gain Controls:
    Most amplifiers will have some sort of level or "gain" control. This control is used to match the output of the head unit to an amplifier. The maximum audio output voltage from different head units will vary. If there were no gain controls, some head units would not be able to drive the amplifier to its maximum power level. Other head units may drive the amplifier to full power at a fraction of its volume control's range.

    A piece of equipment that has a higher maximum output voltage is not necessarily going to sound better than one which is only capable of 2 volts output. The higher output will allow you to reduce the gains on your amp (or any down line signal processor) which will lower the noise floor of your system. If you are not having trouble with noise (alternator, hiss...), you may not benefit from the extra output voltage. You also need to realize that the voltage may be more than your amplifiers can handle. The extra voltage will not damage the amplifier but if the lowest sensitivity (gain setting) on your amp is 2 volts, and you drive it with anything more than 2 volts, it will cause your amp to clip*.

*This assumes that the gain setting is the voltage that will drive the amp to max power. Some amps are underrated and may have slightly more headroom and therefore will clip at a little higher power.

Pre amp voltage is the amount of voltage that the head unit can send through the pre-amp outputs. Most head units are between 2 and 4 volts, and almost every amplifier has a wide enough input gain variability to accept anything within and somewhat beyond that range. The benefit you get from having a higher pre-out voltage is that it will allow you to turn down the input gain on the amp, thereby lowering the amp's noise floor a tad. That's great, except that a good amp will have a very low noise floor to begin with, so lowering it by 3dB by doubling your pre-out voltage is going to basically be a non-issue. Also, the higher voltage that's running through your RCAs, the less noise it's likely to pick up, but remember that the 2 or 4 volts is the peak output (i.e. it's maximum output at maximum volume) and that during the vast majority of the time, you're at a very small fraction of that. In most listening cases, you're probably only pushing 50mV or less through the RCAs, so if you double that to 100mV, you're still talking about very little voltage and a high noise susceptibility either way. Proper routing of your RCAs will have a far greater impact on noise than your pre-out voltage.

actually the use of high output are to stabilize the output transmission and distance...high output = low gain setting are not true......

high voltage will let ur freq to transmit in the cable more stabil, and more freq will be able to reach the amp as compare to low voltage..thus u get better signal amplification...and will have more details of the freq(less freq loss) from the hu output to amp