a reverb does not has its own "sound" if we wanna look at it from a tone generator point of view.
What it does is just sorta process your guitar sound and make it seem like in a hall, in a room etc. That spacey and hollow dimension.
the main tone in the tube zipper clip sound to me like some oscillation or extreme filtering thats going on and the frequency is being manipulated(for first clip) while the holy grail just adding that space behind..
same goes to the 2nd clip, tube zipper, add delay, repeat howling and the holy grail adding that dimension behind..
i cud be wrong thou. But if you wanna get the holy grail for that howling sound, i can tell ya, it wont do it..