I think this started as such:
The "reference level" keith jay asked about is the ability to match the volume used when mastering the audio (or a presentation in a "reference" theater or such) to the level used when listening at home. i.e. if I hear an explosition at a theater and measure it as being 75db, how can I ensure that when I listen to the same explosion at home I hear 75db there as well.
The issue has a bit more to it, though, which is good to understand.
Using a test signal (from an audio setup DVD or CD or such) with a known level (say, 75db as is common on some calibration discs, which was specifically and carefully recorded and mastered as such), you'd want to adjust 1) each loudspeaker and 2) your preamp/amp such that the audio heard at your listening spot is at a level of 75db. This is the calibration Mark Seaton spoke of - the idea is that you're taking your entire system into account (CD/DVD player, interconnections, preamp, amp, speakers, room) to ensure that 75db as encoded on the test disc results in a 75db signal at the listening point.
When this is achieved, the volume position of your preamp/amp could be considered the "reference level" for your preamp/amp. It has _nothing_ to do with
where your volume knob/level is. I've seen preamps where the level was about -15 degrees from top center. I've seen preamps which said it was at -6db. Does the position/value mean anything? Not really. Is the position significant in that it represents a calibrated state? Yes.
Remember that the preamp/amp are only single components in an audio system (which beings with the recording and mastering of the disc, and ends at your ears) - the number/position of the volume only states the state of calibration to make the preamp/amp fit in with the rest of the chain.
Someone please correct me if I missed something...
-Jon