If I remember correctly, we already used a pruned sound bank back then, but I'm not 100% on that; I do know that we had 2 different sound banks; not sure if that was just because we were replacing the original WC3 ambient tracks. (Note: I don't think you actually have to do that. MIDI sounds are still sound handles, you can create them from any midi file, it doesn't have to be one of the preset environmental sound filenames. Although I'm not sure how it chooses which sound bank to use, is that written in the MIDI file?)
The problem is that the midi files we have seem like direct conversions of the original soundtrack that uses a full orchestra, so pruning didn't help much because the bank still had to contain a bunch of instruments. If we had a low-instrument version of the theme (like a piano version or something like that, made to be played on one instrument), we could use a custom low-quantity high-quality bank.
BTW, I did a quick google search and found something better. At this link
, the #1 theme sounds to me like what we have now, while the #3 and #4 versions of the theme sound much better.