Bob you are correct. I use a free Stylizer program from Ntonyx.com which applies algorithms to the Midi data using the GM instrument number as the base together with a small number of variable parameters which has a good stab at improving the musicality of the Midi data.

This is not perfect and some editing using the Piano Roll is usually necessary. Even then much of the clever stuff is difficult to produce authentically. I have also banks of soundfonts which include falls, doiks, stabs, growls etc. which can help. However, the end results do not fool a musician.