I am not a BIAB expert but I'll try to help. The midi sounds you hear coming out of your 15" Peavy speakers is only as good as the midi sound module you are using. Not all but most stock sound cards that come with a computer sound like you have described, especially older PCs. One cure for this is to use a software synth that probably came as part of the Mega Pak you purchased. That is the Roland VSC3. PG Music also sells the Forte soft synth .
Yet another fix is to use an external hardware module like the Roland Sound Canvas. Another good hardware synth I can vouch for is the Roland XV-2020. There are many good general midi synth modules out there . You will have to so some homework.

Another option that you have is to utilize the Realtracks and get away from midi entirely. You can render a midi track to a real Tracks instrument or you can pick a Realtracks Style and most of the instruments in your midi or BIAB file will sound great.
I hope this gets you pointed in the right direction.

http://www.saxmagic.net