I find a richer sound by mixing instruments from different libraries. (I wish I had more.) You also stand less chance of sounding like other composers by mixing libraries rather than playing exclusively out of one library.
This piecing together must be done on an instrument-by-instrument basis. For example, don\'t play an instrument (serially or layered) in your mix if it has a dominating characteristic ambient that is unsuitable for the work or clashes with the other instruments.
I find that AO, Ultimate strings, and Quantum Leap Brass go very well together, with some exceptions. I also use many free sounds from Worra\'s site as well as free soundfont sites. Of course, some of the instruments at the free repositories are useless (maybe as many as 90 percent) but the remaing ones can be tweaked and worked in.
I agree with Clueless regarding mixing various libraries. Each instrument has it\'s own character and timbre in an orchestra. I also detune similar instruments which fills out the sound even more on chordal passages. No orchestral instruments are always in tune - this is why ensemble samples sound more realistic on unison lines as they are slightly out of tune with each other.
I am guilty of playing multiple notes per patch to save space on my sequencer when I really should be treating each patch as a single instrument. The latter, of course, gives individual control over MIDI parameters.
I intend to try separating instruments more in the future. Do you have a strategy or procedure for de-tuning or re-tuning for multiple instruments (e.g., brass or strings)?
I only detune the brass and woodwind sections. The brass are usually detuned about twice as much as the woodwinds. I do the detuning in Cakewalk as the event in the track using pitchbend. Brass detune good from -150 to 150 and woodwinds from -75 to 75. I don\'t know how this converts in other sequencers. In the trumpet section for example, the 3rd trumpet may be detuned -150, the 2nd trumpet 150 and the 1st trumpet is in tune. This varies from piece to piece depending on who has solos and the samples used. Some samples are slightly out of tune to begin with - so I don\'t normally detune these. This technique has really improved my brass sections when playing chords. They get a richer chorus sound more like the real thing. Now for some better brass libraries!