To get better sound from MIDIs in general, you should think about replacing the default Roland soundfont (small sound canvas) with one of the many free soundfonts on the internet. Two good sites for soundfonts include hammersound and personalcopy . The largest problem Mac users will face when trying to use soundfonts is that they are compressed in SFPack or sfArk formats, formats specifically designed to compress soundfonts. These programs are PC-only, so you'll need to use Virtual PC to decode the files. However, I have downloaded the Reality [20mb download], Unison [20mb download], and Musica Theoria [24mb download] soundfonts, unpacked them and stuffed them into a Mac-friendly .sit format. Click any of the download links above to start the download.
Once you've unstuffed them, place them in the directory /Library -> Audio -> Sounds -> Banks. Then open up the Music tab in the QuickTime preferences panel, choose the new soundfont, and click on 'Make Default'. I haven't found any non-commercial soundfont that suits all music perfectly, but these do a good job without becoming too large. Though Unison suffers from some downtuned instruments, it is what I use for most my MIDI files because of the great choir, distorted, and overdriven guitars.
To easily compare the quality of these soundfonts to the small default quicktime font, I exported a MIDI to mp3 using the sound canvas and unison soundfonts:
Eagle-Eye Cherry - Save Tonight:Another way to raise the quality of MIDI music files is to export them to another format, say AIFF. When exporting, QuickTime outputs the samples at higher quality, producing crisper, fuller sound with a better stereo image. Compare the three files above; the exported files will sound better than straight playback of the MIDI files.

