Correct statement would be: 'Recent versions of WadSmoosh work only on GZDoom (or QZDoom); they do not support old ZDoom without few extra actions'.

For example, WadSmoosh release dated 2016-08-11 works perfectly even with ZDoom v2.7.1. Unfortunately it looks like author of this very handy tool provides only the latest version as a complete download for the end users.
So, the easiest way to get old ZDoom compatibility is to open the pk3 built by the WadSmoosh with your favorite archiver and remove menudef.txt lump. Visit official issue tracker for the details: https://bitbucket.org/JPLeBreton/wadsmoosh/issues/11/backwards-compatibility-with-zdoom#comment-42600363.
Alternatively, you could extract one of the former snapshots from the official repository. But that's is definitely a hard way, which requires some technical experience. And latter improvement of the code would be lost as well with such an approach.

I found Timbres of Heaven caused distortion when used for MIDI tracks. The best soundfont I've used so far has been Musyng Kite, detailed on this SoundFont webpage. The downside is that it is nearly 1GB when expanded to .sf2 format and it requires the use of SFPack (available from the same page) to extract.

With GZDoom, I did find it resulted in very slow framerates when hardware (graphics) mode was used, but exiting and restarting GZDoom seemed to fix that - just in case anyone else encounters a similar issue.

And for those who want more still, the full Musyng expands to 1.6GB....