A pal of mine did this when he upgraded the speakers in a used Marshall cab that he bought, and he told me that this helped out tremendously:
[youtube=425,350]http://www.youtube.com/watch?v=fIRffx48KVc[/youtube]
Even with hemp being stiffer, a low-volt AC load should will allow it to buzz at a constant frequency, which will loosen things up, but not shake the speaker to bits.
Personally, if you are going to make a speaker called the "Cannabis Rex," you really should design a more unique label to affix to the back of it, in maybe more of a ganja motif rather than stars and stripes.