I didn't think you had modified the cone, but it is a fairly common practice regularly discussed on other forums. A popular modification is to apply material to coat the speaker cone. I've done that a few times, but only when I was very young. I tried model airplane dope, paint and glue - Sometimes for looks and sometimes to change the cone's weight and stiffness. But I realized pretty quickly that doing stuff like that by hand wasn't very consistent.About the plastic horn of the tweeter, I'll have to cover them with clay or tar or something and test it out. The midwoofer energy probably excites the plastic just a little bit, so maybe adding something like this will deaden it. Honestly, I've never noticed a sound from any of the plastic horns, but it sure can't hurt to cover 'em with clay and give it a go.