I have some older MIDI backing tracks that I want to update to sound better when they play through the PA. The software I'm using allows me to set the level, velocity, pan, reverb and chorus for each instrument.

An addition to the original MIDI specification that allows for consistent playback on any instrument that is GM compatible. Achieved by requiring that groups of sounds have very specific locations (a piano sound on one synthesizer will have the same location on another,) that all instruments have 24-voice polyphony, and that certain continuous controllers are similarly defined.