Mark,

I'm not sure if I am with you.

The chain is to my idea always the same.
Composer sees the Midi-port driver, that connects to a "midi sound generating device", internal or external.
The midi-sound generation device requires a device definition.
Am I wrong here?

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Examples:

In case of the TYROS their is a USB Midi driver,
that presents it self as MIDI Port Device. It's however specific to connect an external Yamaha's USB Midi device, could be any keyboard/synth from Yamaha.
The external device needs a device description regards voices, rhythm etc. so Tracks get right Instrument names and patches.

In case the MIDIsport USB device, its exactly the same, but universal.
This USB device needs a driver that acts as MIDI Port.
You may connect any external Midi device, that again requires a device description.

IN case of using a sound card with midi connector. Also the same
The sound card drivers present one midi port, to which you connect
an external device, what ever brand or make - with a device description.

In case of software like Microsoft GS or Yamaha XG wavetable, these presents themself as "Midi Port, and attached to it the "wavetable generator" which requires a device description.

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Cheers Jaap