In order to clarify the topic and add some data, I confirm that you can get the best GM synth of the world, but if you don't dive into the MIDI data , you'll never get a correct final result. Many people think first about 'sound' because it looks easier to change of sound module. But as said Jford and MarioD, if you really want professionnal results you have to go in some MIDI editing. For me and for many home musicians, it is a necessity. But I admit it take some time, it need a good understanding of MIDI controllers and good notions about instrumentation. In fact, you have to work on two aspects of your music : #1 the sound itself (that is the GM MIDI engine) AND #2 the music stuff (that is arrangement of your song, editions of CCs, stylizations according the styles, ...). So, some work for a long time....


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