You are quite right Fran. With loading and saving in NX-mode, you can use more than the 128 sounds that GM standard describes.
Some remarks however:
Loading in GM-mode ought to give the most accurate sounding (not the best sounding) song as compared to the original recording.
Volumes levels and the choice of sounds are standarised, therefore the midifile should sound the same on different keyboards, sound-modules or (computer) soundcards. This is definitely not the case, as you obviously notice when playing the same midifile on your computer and then on your keyboard.
You can load a midifile in NX-mode and change settings as you can when loading in GM-mode. I use to load a midifile in GM/GM2 mode and first listen to it. When I'm satisfied with the quality of the arrangements and decide to use it, I start changing the sounds to my liking, as I do with volume levels, reverb and other controls.
When I'm all done, I save it as a Technics file, indicating that it is a modified midifile. Saving as a midifile might give strange sounding on another soundboard.
For detailed information on the GM and GM2 specifications, see the website
http://www.midi.org/ GM2 is an international standard of a tone generation format, designed to achieve greater musical expressiveness and hardware and software compatibility. An enhancement of the previous GM (General MIDI) standard, it defines detailed operational specifications for voice editing and creating effects not covered by its forerunner, as well as expanding the number of available tones.