Back in the day, early nineties, I use to collect a lot of MIDI songs on my MS-DOS/Win3.11 PC, a 66MHz Pentium 1. I played a lot of games and liked to remix MIDIs as well, but it was only beeps and bloops.
Then I installed the PC Version of "Final Fantasy VII", which included a free version of a Yamaha synthesizer app that improved the MIDI instrument quality. To my surprise, it improved everything the computer played with MIDI. I went back to my remixes and they sounded like orchestras instead of bleeps and bloops. I played "DooM" and "Space Quest IV" and was blown away by the music quality.
Now, opening the same MIDI files on my maxed-out 2014 iMac, I was hoping for quality the same or better than my old Pentium after it's update. I think the quality might actually be worse.
Like on my old PC, I'd think there's a way to update the instruments system-wide and still use the same apps (Quicktime/Garage Band) to play the files, but I haven't found a way to do so yet.