I'm no electrical wiz, but I'll try. Someone please chime in if needed...
The sources that you can use (your cd player, DVD player, phono, tuner etc.) will put out different maximum voltages - CDs players typically put out 2 volts max (some more). Some modified digital sources put out less, maybe 1V. Perhaps you may have some phono setup that puts out less, etc.
These sources often get put into a preamp (can make the signal larger/quiter by amplifying/attenuating it) or a passive linestage (no active circuitry and can't output more voltage than it gets, only less) (OK there are many variations but trying to keep it basic).
Then comes the amp - Let's say you have two 100 watt amps, equal in every way EXCEPT for the input sensitivity. - Amp 1 may put out its full power with only 0.7 volts being fed to it. Amp 2 needs 2 volts to reach its full output. As you can imagine, the volume produced by these two amps will be very different if they each recieve the same input and use the same speakers.
Then the speakers - some speakers (efficient ones) make much much more sound than others given the same amount of power from the amp.
So what does this mean in the real world? The 2 extremes :
Too much gain - If your source has a high output, you put it through a preamp with lots of gain (lots of ability to increase the level of what it's fed) and then amps with low input sensitivity and efficient speakers, you'll only be able to turn the volume knob a very small amount and it'll get loud real fast and you'll say "Hmmm, I can only turn the volume up 5% of the way - seems silly that I can't use most of the volume range that I have"
Too little gain - Low output source, no increase in voltage from your preamp, amps that need lots of input voltage to reach full power, and speakers that need to be driven hard to get loud. "Hmmm, I got the volume at 100% and it's still not loud enough!"
So in my previous post, with amps that need only 0.7 V input to each full output may give you some flexibility to use a preamp with little or no gain, or sources with lower than typical output level.
I hope I didn't make it seem like an impossible task to match the various pieces in your rig! I'd guess that many folks use only the lower third of thier volume setting with no problem.
Knowledge of the amount of input voltage your amp gets and how much voltage it needs to reach full power can help to properly match the other pieces of the system. (Or, the characteristics of the rest of the system can sometimes make the amp's input sensitivity a factor in amp choice)
I'm not saying that all this sensitivity stuff can determine which stuff to buy, but will help to avoid the 2 extremes mentioned above.
Hope this helps!
-Mike
