It seems I have had a long career in orchestra performance. I believe I have worked under the baton of over 100 conductors. Those that I consider most successful start with a warm-sound approach. They lower the base sound of an orchestra to a warm mezzo-piano. Starting with this warm soft dynamic a conductor can achieve wonderful blends of instruments and tone qualities. Then the musicians when needing to play loud have a nice soft base to build upon, and the contrasts can be huge, and at the same time the loud can be non-forced, but beautiful.
If a conductor were to start by saying "I can't hear you, play louder," which is what most conductors do, we generally have an escalation of volume and a loss of warmth. Yes, there are times when the expression of the music needs harshness, but to start with a sense of beauty makes the music much more emotional.
I have seen a few conductors also start the rehearsal with creating the phrases that he or she feels in the music. This is a fantastic trick, or tool that a conductor can use. Step one is reduced the volume to find warmth and blends. Step two is suggest a few phrases. Once the musicians are listening for blend and approaching the music with warmth in their tone, the phrasing of the first two phrases carries through the piece and with two simple approaches the orchestra starts to sound as if they have music in their souls.