first you need to connect qtractor midi out to your outboard synth midi input--i guess you have this setup working--then getting your synth audio output into some line-in of your soundcard and then recording it into an audio track would be my suggestion.
get midi played by qtractor be sent to synth: qtractor (midi track -> midi out bus) --(alsa_midi)--> m-audio fast track pro (midi out) --> yamaha s90es (midi in)
get audio played by synth recorded into qtractor: yamaha s90es (audio out) --> m-audio fast track pro (audio in) --(jack_audio:capture)--> qtractor (audio in bus -> audio track)
first you need to connect qtractor midi out to your outboard synth midi input--i guess you have this setup working--then getting your synth audio output into some line-in of your soundcard and then recording it into an audio track would be my suggestion.
qtractor (midi track -> midi out bus) --(alsa_midi)--> m-audio fast track pro (midi out) --> yamaha s90es (midi in)
yamaha s90es (audio out) --> m-audio fast track pro (audio in) --(jack_audio:capture)--> qtractor (audio in bus -> audio track)
hth.
seeya