Even though Operator is a very powerful synth, I doubt you will get more realism than with samples. It might be more expressive, but not more realistic. I'd suggest better samples or a physical modeling instrument like the one from Arturia called Brass.
http://www.arturia.com/evolution/en/pro ... intro.html
It doesn't come cheap, though.
And for the bass presets part, I can't really help, I don't use them but I'm sure there's plenty of that around. Bass is fun and easy to program, especially with Operator. You will get more satisfaction out of programming your own sounds.