I think the real key to realistic brass or any wind instrument is in the envelope and application of noise. Everytime you're working on the amp or filter envelopes, it's important to not let yourself forget that you ultimately want the complex sound of blowing into a warped metal tube
Surprisingly its not as complex to achieve via programming as the sound itself is! But a synth with complex filter possibilities is a great tool in the tool box -- perhaps more importantly is your own familiarity with the synth (or at least that's what gets in my way the most.... I tend to create the best sounds with whatever synth I've invested the most hours with, regardless of the quality of the synth itself).
Hate to say it but sometimes for brass sounds you get the best results from out of box samples or romples, but you can achieve same with some filter work then finding the harmonic content you want.