can you provide an audio sample of the current mix? It might be easier to help then.
There are two absolutely fundamental techniques that you may have already tried, but did not mention:
1. adjusting the wet-dry mix.
2. Using power-panning, reducing ALL solo instruments to mono. Then, place the mono signal in the desired stereo position.
A third trick is less important, but based on what you said, it may be relevant:
3. Using EQ to simulate "bass loss" on many of the instruments, particularly brass. Personally I found this was more important than reducing high frequencies. I can't remember exactly, but I think my generic brass EQ is starts around 800 Hz - 1 KHz, and goes around 3-4 dB per octave to reduce the bass. doing this also helps the woodwinds too, especially the flute.
Do any of these suggestions help?