As for standing waves, you're right about doing the math. No matter how it's folded, if you have three sections of approximately three feet each, I'd expect the horn to work well up to about 100Hz. So as long as output is limited to 100Hz, it should be alright.

Still, one thing that's attractive to me about the equiangular spiral is that the radius is constantly increasing. When a horn is folded to approximate this shape, each section's length is different and that tends to break up the standing wave modes along the long dimension.