It was always my impression that an amplifier is designed to deliver its maximum
power at full volume. This is different to perceived
loudness, which is a combination of amplifier power, loudspeaker sensitivity and the loudspeakers' distance from your ears. And this doesn't take into account the subjective opinion of how loud you like your music. If you simply prefer your music at a certain level then it doesn't really matter where the volume knob is pointing. If, however, you get the volume knob half-way around and the sound is compressed or distorted then you have reached the limit of your amplifier's output. In this case it is most likely the input signal is higher than the amplifier is strictly designed for. Not a big deal, just turn the volume knob down. If the sound is
always compressed or distorted then the input stage is being over-driven and you'll need to adjust input sensitivity or use attenuators. Also consider that different sources typically have outputs at different levels. As mentioned previously, CD and DVD players (analogue outputs) are usually 2V rms, while most other devices stick to a standard 0.5V or 0.775V rms. You would hope manufacturers are aware of this and would design their amplifiers accordingly - i.e. design the 'CD' input with one quarter the voltage gain of all the other inputs - but you never know. And if your amplifier pre-dates the CD revolution then there's no chance it'll cope with a 2V input on its own.
Tractorboy raises a valid question - will attenuators affect sound quality? Yes. Probably. Inserting anything into the signal line will have an affect on the sound, but whether you notice it will depend upon your system and your ears. You will be adding one extra plug-to-socket joint, a few solder joints, and a resistor or two of unknown manufacture. It is also conceivable that an attenuator could introduce noise into the signal, but I'd hope to see it built into a shielded metal can (like the Russ Andrews ones). However, there shouldn't be any adverse effect just because of its resistance as line-level signal current is negligible.
As to what volume position should
sound best... it's all starting to get a bit silly. And I don't think you can generalise - turning an amp up puts greater demands on its thermal efficiency and the efficiency of its power supply, while keeping it quiet puts greater demands on the design of the amplifying stages (immunity from noise, crossover distortion). Where did the manufacturer spend their money? If you know this then you might be able to find the perfect volume position. And then superglue it in place
I'm trying not to write an essay here (and failing). On the one hand you have pure physics and, on the other, audiophile voodoo. The truth is probably somewhere between the two.
And yes, Chris, I play bass, but probably not well enough to have three nice basses! I have two obscure cheapos and a Warwick Corvette 4-string. I have it on good authority that it's the only Warwick that doesn't play like a pig!