I have not seen any value in using "Cache BB Code output" and have it unchecked on my forum. You can also empty the table that holds the cached data.
1) Uncheck Cache BB Code output.
2) Using phpMyAdmin empty the xf_bb_code_parse_cache table.
Once you have done this, you will see that editing the xf_post.message can be done on the back end and when you reload the page in the front end the post will show the changes you made in the back end.