The description you give makes me think of the sound of a compression horn that is pushed past its limits. They get rough when overpowered or used at too low frequency.

Of course, the source signal can do this too. When the amp clips, the sound that results is similar. And I've heard it on some 60's and 70's recordings. A great example is "Dear Diary" from the Moody Blues album, "Threshold of a Dream." You'll hear this sound on the best system in the world. Ironically, it sounds better on a mediocre system.

The engineer must have overdriven the panel or tape when he mixed down the master, because it isn't present on the greatest hits album, "This is the Moody Blues." I notice that the effects vocal is reduced on this song and on the poem "In the Beginning," also originally from "Threshold of a Dream." So I guess the mix-down engineer caught it second time around.

Wayne