Nigel.
I would not even consider Coolsoft as a "SYNTH" it has it's place but not as an instrument.
For GM, Halion Sonic is probably most stable and advanced of all the software GM synths I tried. My next choice is VST SynthFont. It is a very advanced "shell" that you can use with separate SF2 fonts or complete banks (free or paid) to assemble your "own" GM bank. At the end of the game it will sound as good as your samples are smile