Some receivers do use a scale from 0 to X, generally termed 'absolute' volume display as opposed to 'relative' (-infinity to +whatever). Onkyo states in their manuals that 0dB on the relative scale corresponds to 82 on the absolute scale.
The scale on one receiver is not comparable to the scale on another, which is why you can't compare receivers by what number on the volume scale achieves the loudness you desire (LOTS of posts like that on every forum I read and participate in - my old XXX was loud at 50 but I have to turn up the new one to 65 to get the same level...).
The scale is meaningless until you calibrate to a specific level and thus have a concrete output SPL level. If you calibrate to a level of say 75dB at 0 on the volume scale, then -10 does have meaning because it means your ouput SPL will be 10dB below 75dB (or 5 dB for the Denon that says volume increments are .5dB). If you haven't calibrated to a known level, then simply saying I listen at -40 means absolutely nothing unless you know what that number means in terms of the receiver's electrical characteristics (and they don't tell us as I said before). I think THX certified receivers do have a specific output at 0dB, but I don't know what the spec calls for.
So in short, volume scales are meaningless until you calibrate to a known level. If two receivers are calibrated to the same level, then they can be directly compared, but the number on the scale is still meaningless. For example, I have my Onkyo 502 calibrated to 75dB at 60 on the volume scale (it uses absolute 0 - 80). If you calibrate the Denon to 75dB at 0, then the two are putting out the same SPL. If you then play the Denon at -10, you are listening at 5dB below your 'reference' of 75dB (if it does use .5dB increments/decrements). If I play the Onkyo at 55, then I am also listening at 5dB below reference. Because we both used the same reference, the output is identical.