I've built a website to document ragtime piano and ragtime orchestra music from the ragtime period 1900-1920 with about 200 piano compositions and 100 arrangements for 10-12 piece orchestras. Each tune is illustrated with a music sheet pdf and a mp3-file, created in Sibelius 7 and its built in sound library Sibelius Sounds plus Garritan Jazz And Big Band 3. I use all the tricks available in these programs, but fail to produce reasonable recordings - the trumpet sound is flat, the strings uninspiring etc but so far I haven't been able to do better. Compared to the sounds produced by most people on this Forum it's simply too mechanical and unnatural (the piano sound is perhaps acceptable) - you can see for yourself on http://www.ragsrag.com if you are interested.
My question is simply this: what hardware/software do I need to get a more realistic sound? The only possible solution for me would be to postprocess the Sibelius files that I already have - not to start from scratch. I've put in about 30 months in this project already and I'm doing it as a hobby, not for a living. My computer is really powerful and I'm on Windows 7 64-bit.