Changing MIDI synth

Hi, everyone.
Today I've batch-converted multiple MIDIs to WAV files with SoundConverter, and I've discovered that it's fast and reliable.

So now I would like to try the experiment again, but this time I would like to select/choose the MIDI synth used for the conversion; and I haven't been able to determine how.

Is this possible at all?
Is there any configuration file I can edit, in SoundConverter? (I think this would be the easiest way)
In alternative, is there any system setting I can tweak? (My machine runs on Ubuntu 14.04.3)

My best regards,
Massimiliano
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