I'm a high school lead trumpet player, so I've been working a lot on range and endurance building chop exercises. They have been working well, and my range has really come along, but now I'm finding my tone quality in the low and middle register to be sub-par. My tone in the upper register is great: clear, bright, focused, and loud, but my tone in registers lower than that is usually airy, especially when my chops are tired (it sounds okay when I haven't played high yet). What are some good exercises to clear up and sweeten my tone quality? I tried long tones for about four months and I saw no effect on my tone, although it helped my endurance.
I play on a Marcinkiewicz E12.4 (Roger Ingram) which is fairly shallow, as I am a lead player, but it's not too shallow that I shouldn't be able to get a good sound on it as well.