I found an interesting essay online (pdf file) all about Keats's gravestone and especially the symbol he chose for it--the broken lyre. Here's the web address where I found the article. You should be able to open it OK if you have pdf capabilities.
http://www.kingmixers.com/Franklin%20PD ... hePoet.pdf
Anyway, I know that a lot of folks don't really like to read scholastic-type essays about Keats, but this one is really interesting and not "way out there" like some essays are.