Every musical track, speaker system, and room has a perfect volume level. It isn't based on the time of day or the listener's mood. It's based solely on what works. And that is something rare.
The Mahler 3d really only works at one level. Too little and the quietest passages are missed. Too loud and the blaring horns are too big. But just at the right volume, everything works. For that recording. In that room. With those speakers. In those circumstances. On that day.
Everything else is wrong.
When we go to trade shows (and by the way, we have signed up to be at Axpona in April) we train our system operators what the correct volume level is for each track. But that level varies depending on the number of people in the room. So, this idea that the perfect level is a simple number is only correct for a given set of circumstances.
It takes a well-trained ear to know what to set it at.
And no, you cannot achieve this with a simple amplitude meter, something I understand will panic those who can't stand the idea of trusting their own hearing, but it's a fact. Once you're aware of this fact, it's not hard to train yourself to get it right.
I've put together a video on the subject you can watch here. Something went amiss with the stupid microphone on this video so please forgive its awful sound quality.