Without calibrating the receiver, the 0 dB position on the volume dial has as much significance as any other - none. [That is the point mentioned by malvado78]
This can be confusing because '0 dB' means different things when you are talking about digital audio levels vs volume settings.
Digital Audio
As most are aware, digital audio is just a sequence of numbers (samples) that represent the amplitude of the signal at points in time. The range of the values depends on the bit depth. As an example, CD audio is 44.1 kHz / 16 bit, which basically means for every second of audio there are 44,100 16 bit samples. The samples are signed (the waveform has both a positive and negative component) so there are really only 15 bits (the high order bit is the sign).
0 dB is maximum amplitude and is known as 'full scale digital'. With CD audio that means a sample of value of -32768 or +32767. When people speak of clipping (NOT amplifier clipping, which is different), it means that there are N maximum sample values in a row. It is indicative of the fact that the input signal was probably too high and could not be fully represented with the available bits. There is no headroom in digital audio - if the signal would be greater than the max, it gets set to the max.
Volume Levels
First, the volume control is actually an attenuator. When the volume is at max, there is no attenuation - the full rail voltage is being supplied. When you turn it below max, the signal is attenuated. If the receiver says that it has db accurate volume control, then turning it down by 1 dB will do just that - reduce the voltage by 1 dB. What that means in terms of output SPL can't be known unless you have calibrated to a known level.
Using 0 dB on the volume scale as 'reference level' is purely a convention. You can use whatever you want. We need to distinguish between 'reference level' which commonly refers to the output SPL and 'reference volume' (my term) which is the number on the dial. Dolby Reference Level is well-defined and it is 105 dB peaks at the listening position (+10 more for LFE).
So here is how that is tied to the above explanation of digital audio levels:
The test tones in the receiver output pink noise that is 30 dB below 'full scale digital'. If you want Dolby Reference Level, you need to pick a setting on the volume dial and adjust the channel trims so that the SPL meter reads 75 dB. Doing so gives you 75 dB with a -30 dB input signal. In the moments that the input signal is full scale (0 dB) you would have the 105 dB peak - 75+30=105.
If the test tone you are using (AVIA) is -20 dB then you would calibrate to 85 dB because again 85+20=105.
You can of course choose to calibrate lower because true reference level is quite loud. Alternatively, if you do want to calibrate to true reference level but the loud test tones is really bothersome, then you can just pick a point lower on the volume dial and pick a lower SPL reading; for example you want reference level at 0 dB on the volume scale, but you set the volume to -10 and then calibrate to 65 dB on the meter (if test tone is -30). You now have 95 dB at -10 on the volume dial and 105 dB at 0 on the volume dial.
[Note that none of this has anything to do with passing the cleanest signal possible, full dynamic range, etc - all of that is solely determined by the recording itself].
