Hi All
The GM is a Midi Standard that allows different instruments to work together with common voices, i.e.: No 1 is always a Piano etc, how the manufactures implement it is up to them.
Wavetable is just one type of synthesis, and is not used by all instruments; in fact if there was enough memory and hard disk space, the GM voices could all be Giga Samples if the manufacture so chose.
The main reason most of the GM sounds are rough, is because manufactures know that people will change them as soon as the General Midi file is loaded, to suit there own instruments, so consequently they spend little money on them

Bill