1. Use the trio approach - have students match the energy and quality of the person on either side of them, so that no one sticks out.
2. It may be that the students are using lots of air without good embouchure's. Something I did not do until later in my career was to play our fundamental exercises at louder volumes while marking time. If I asked them to play louder in the show- I never wanted them to play louder than we did in fundamentals. For me- a two count breath and making sure they stay relaxed (relaxed tones, relaxed shoulders, relaxed hands, etc) allows students to play loud and have impact without being spread. It is not easy- and it takes time, but it is worth it. My two cents- I would rather hear gorgeous sounds that are not as loud than to hear loud sounds that are spread. Hope this helps and best of luck with the rest of marching season.