Interesting discussion.
On my system, it sounds very good around 60db, that is usually my morning listening volume while the house is asleep. The sound is clean, clear, good soundstage, good dynamics.
When I turn it up, it does get louder, but it does more than that, the soundstage gets much bigger, everything opens up, get better imaging, with the bigger soundstage, much more presence. Everything is clean, more clarity, better seperation. The music has weight to it. You can feel it pressuring the room. This works up to around 100db.
Over 105db, it's still clear, but starts to hurt being in the room, the soundstage is much bigger than the room. But there is no distortion, you can still pick out each instrument. Mids do start to get a big forward, the highs can start to get harsh. Bass however is glorious! The whole house starts to vibrate. At these levels, the treble control goes from 0 to -4 or -6, to tame the harshness.
The sweet spot is around 90db at my listening spot, everything just goes tougher.