Q: Softsynths and General midi

[zigbert]

I have trouble to get softsynths playing my GM/GS midifiles correct. Well, - with some tweaking the synth plays correct, but the instrument number doesn't seem to match the general midi protocol. This tweaking differs from synth to synth. - WHAT HAVE I MISSED. I'm sure many softsynths plays GM midi-files out bof the box.

I've tried the following synths:
AMsynth
Hexter
HX44
RTsynth
Xsynth
ZynAddSubFX

I see that some of these are specialized synths, but ZynAddSubFX and Xsynth has a wide instrument list.
I use Muse.

Thanks for any reply.