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....

Although it's not technically Doom, these games do use Doom's engine, so most people here will find these links useful:

https://www.gog.com/forum/heretic_hexen_collection/heretic_and_hexen_1_sourceports

https://www.gog.com/forum/heretic_hexen_collection/running_hexen_deathkings_of_the_dark_citadel_in_chocolate_hexen

https://www.gog.com/forum/heretic_hexen_collection/mods_for_heretic_and_hexen

Anyone looking for sharper, higher quality textures for Doom should check out this thread:

Doom Neural Upscale 2X [v 1.0]

While this can be played by dragging and dropping the .pk3 file onto gzdoom.exe as suggested in that thread (only if you use Windows Explorer - it won't work with other shells), a more convenient approach is to create a new shortcut to gzdoom adding the following parameter:

-file NeuralUpscale2x_v1.0.pk3

(this file needs to be in the same folder as gzdoom.exe).

Something I want to throw your way Phil for those looking for a faithful experience similar to Chocolate Doom or Crispy Doom but with the added convenience of controller support, built-in MIDI playback via Fluidsynth for *.sf2 soundfonts, and HD resolutions with the old-school look, I would highly recommend Woof! and Nugget Doom. Since these ports are based on the software renderer of Doom, they don't require the amount of work to make the game look faithful to how it would look in DOS or Crispy Doom that GZDoom does with its ugly default settings. They also support voxel packs too, and combined with the original low-res look of the game, the voxel packs can create this cool optical illusion when viewing the enemies and items from afar where they look low-res like the DOS version, but close-up you can see them in their voxelized glory. Links for these as well as the SC-55 soundfont can be found below:

Woof! source port: https://github.com/fabiangreffrath/woof

Nugget Doom source port: https://github.com/MrAlaux/Nugget-Doom

Grieferus's SC-55 Soundfont:https://drive.google.com/file/d/1pLyodRR5NKh-wric9ntho72HfNm3wjZz/view?usp=sharing

Voxel Pack: https://www.moddb.com/mods/voxel-doom-ii/addons/voxel-doom-ii-with-parallax-textures (Made for Doom II but has been updated recently to work with Doom I WADs.)