The dB on your volume control is very different to the dB your speakers will make.
For starters you need to look for the efficiency of your speakers, typical bookshelf speakers are often around 86dB/Watt @ 1meter, on the other hand typical floorstanders can be 90dB/watt@1meter or even higher.
Cerwin Vega are famous for 'Loud' loudspeakers as are JBL, both making speakers with efficency > 100dB/Watt@1meter. But loud doesnt normally equate to quality.
Now, to double the sound level, you need to increase dB by about 10, however to increase dB by just 3 takes 2x more power from the amps. Also the speakers are rated at 1m. For every meter you move away from the speaker the volume is reduced. Approximatly 6dB per meter.
So if you sit 4m from those 100dB/Watt Cerwin Vega's you'll be getting around 76dB's, and to get back up to 100dB from your listening point would take around 250W of power.
(Note these are fairly generalised figures, to give an approximation, I cant be bothered to lookup and do the exact math
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Thus the floorstanders in my example would be considerably louder at any given volume on your amp.
Think of your amp as two separate componants, the power amplifier, and the pre-amplifier. Audiolab power amps (actually many other amps too) have a gain of 29dB which means they convert a 1V input signal into a 28V output signal. (Would require 100W to produce the full 28V signal feeding typical 8 ohm speakers)
On the other hand your CD player probably has a 2V output, so if your connected the CD player directly to the poweramp, bypassing the preamplifier, well, it would be bad
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So your pre-amp is often used to attenuate the signal level from the source to a more comfortable listening level. (with the exception of a turntable, where huge amounts of gain are applied to the tiny signal from the moving magnet, or moving coil cartridges.)
When your pre-amp is at 0dB, its feeding the poweramps with the full signal from the source.
My own amps Audiolab 8000M are able to push out more than 100W, on paper they are rated at 125, in reality they dont distort or overheat even at 140W (put them on a scope, lovely clean signal, no clipping etc). So I can safely push my preamp into +dB's. My preamp goes from -infinite dB, to +15dB, although I normally find myself running at around -20dB on the preamp giving a sound that fills the room.
Lower wattage power amps often have lower gain, this makes the pre-amp do more work, and also means you can turn the volume knob up higher without overloading the amp. Although obviously they will never be as loud in the peaks as a higher powered amp.
With integrated amps the pre and power amps can be matched in gain and its only the total gain that really matters so if you compare two 100W integrated amps, one with a volume control that goes to 0dB, and another that has a volume control which goes to +17dB, the end result will probably be the same. The 0dB amp will likely have the poweramp stage at +29dB, while amp which has the pre-amp boosting the source may well have lower gain on the power-amp stage. (otherwise its just going to overdrive/distort/clip etc)
Why build integrated with +dB... Well when you go into the shop, the salesman might tell you "Look at this amazing amp, it goes to +17dB, thats much louder than the 0dB of the other amp". In reality of course if your comparing two 100W amps, its just they are built to a different design, maximum power will still be 28V into 8 ohms, so max volume is dependant on speaker efficiency.
Hope that answers your question.
PS 0dB gain does exist. at 0dB gain there is zero amplification. Feed 1V in, and you get 1V out.
PPS, on AV 0db is often used as reference, but then as speakers all have different efficiencys AV amps have extra controls normaly buried in the software to apply further attenuation (or sometimes gain) to the signal as you measure it with a meter. In stereo hifi, its not really used as a reference, its just the attenuation (or gain) in the preamp thats written on the volume control.
(although you have assumed a point source emitting spherical waves in anechoic conditions - a dangerous assumption to make when the speaker is in a room just a few m from a listening position...)