The Ensemble software uses a Soundfont synth to generate it’s MIDI sounds. This is quicker and better quality than most PC’s built-in Microsoft synth. It also means that several different makes and models of PC will sound the same when playing MIDI notes and chords.
If you want to add other Soundfonts to the software, you just need to put them into the folder C:\Apollo Ensemble\SoundFonts. They will then appear in your list of MIDI devices.
Use the ‘Identify’ button to listen to the synth and then click on ‘Apply Changes’ to use the new synth.
You may need to close and reopen any Designer and Player windows you have open.
This website has links to some good Soundfonts: VirtualMIDISynth