Lately I've been finding that under certain circumstances, like when playing really hot and pushing a power amp and speakers hard, the bright switch ON may be a more correct choice. At lower volumes when JBL's are less "pushed", they exhibit an extra crispness of their own so the bright isn't so helpful, can be too much. When JBL's are run loud, the warmer tone from the paper begins to dominate over the crispy sparkle of the dustcap, so bright ON seems to be the right choice.
You may notice that at really low bedroom volumes, the JBL's don't sound the same, you hear TONS of the metal dustcap relative to the paper. That balance between paper and metal dustcap changes quite a bit from quiet to loud.
I believe that the more we can do to really push the speakers to their limits (without blowing them) is part of the sound we all seek. Since that makes things really loud, maybe for some stages the right approach is just one JBL pushed hard. 3 JBL's pushed hard is gonna be loud, especially if it's an E120. D120F's or K120's are easier to push to that magic, musical-sounding edge. And Alnico is a bit sweeter sounding than the E's ceramic magnet. Brutally clean is NOT what we want. We want some subtle dirt, some amp and speaker stress, and we want it at the perfect stage volume. This is the mission and not an easy one at that. Once we attain that perfect amp and speaker stress/soft-clipping, then we're in a better place to determine how that bright switch is treating us.
It's gonna get loud no matter how you slice it.
... and it's just like any other day that's ever been...