Apple has Broken iTunes for Me
I guess it was always a workaround, but I had a nice workflow with it anyway:
1] Compose in Encore
2] Export MIDI file
3] Drag/drop into iTunes
4] Select soundfont bank in QuickTime Preferences
5] Create AAC version in iTunes with selected soundfont
Well, starting in OS X 10.6, there are no longer any QuickTime Preferences, and after 10.7 iTunes 11 won't play MIDI files at all. So, I've had to compose on my Mac Pro (10.7.5/iTunes 11), send the Encore file to my Macbook Pro (10.4.11/iTunes 9) via the HD attached to my AirPort Base Station, and do all of the fiddling there. Yes, a major PITA.
I finally had enough of that noise, so I was looking for a MIDI to MP3 converter. Well, those are a dime a dozen, but none of them allowed for selection of soundfonts, as far as I could tell. Finally though, I found a nice app for the chore that allows me to use my soundfonts: MIDI2MP3.
It's completely intuitive. Less than sixty seconds after opening it, I had a test conversion done.
So, if you need to make MIDI files into MP3's using your own soundfont collection, this is the easiest way. It's a major improvement from even my pre 10.6 workflow: Many less steps.