How does source volume affect an amp?

[KB24]

I want to know what happens to an amp when you lower the volume at the source (i.e. iTunes)? Does the amp work harder to amplify the lower signal? Does it work not as hard? Or is it unaffected?

 

[ProtegeManiac]

The signal volume, in dB, is lower, and if the analog input is the one that's lowered, so is the voltage. Whether the amp will work harder or not depends on whether you need to increase the volume to suit your preferred level. A lower volume potentially lowers the noise from the source, but if your amp doesn't have clean output either, then you'd end up getting distortion from the amp instead. Proper gain match minimizes this, but I'm more familiar with doing it at the analog level even with a digital volume control on either end. For example, in a car system you match the relatively noisy 6.5v output of a Pioneer Premier receiver vs a really clean 1.8v  output from a Denon to your amp with gain controls, you actually just end up using around 1.8v to 2.0v off the Pioneer. The draw of Alpine and Pioneer over a Denon are the sound-tuning options (think Rockbox vs S:Flo2), but of course Pioneer one-ups Alpine in sales by having an advantage that bassheads who want their woofers audible from several blocks away would want.
 
For example, I was matching my Rockboxed diyMod 5g and S9 to my Toucan, and in both cases a too low volume in the source (to prolong batt life) compensated for by the amp results in a thin sound. At full volume at the source, I can't go beyond 9AM on the Toucan but no matter how thick the sound is, the bass is flabby and the HD600 drivers distort with some notes. I find 35/40 from the S9 and about 25% on RB's volume indicator bar work best. Both were set with no sound shaping options enabled.

 

[khaos974]

If the source has some noise, you would be amplifying the noise too.Assuming the noise level at the source is constant, increasing the signal at the source increases the signal to noise ratio, which is a good thing.
Volume at the source should be set to max.

Caveat 1: check what the max input level of your amp is, and what the max output of your source is as far as voltage is concerned. After that set the volume of the source as high as possible without going over the max input level of your amp.
Caveat 2: if the input level is too high it may result into your using the lowest range of your amp's potentiometer, some of them have a lot of imbalance at low level (say at 8:30 for example).