same problem here. i have a tf300t and can only access apx interface.
I got into this mess, after by "accident" choosed the "wipe data" option present on the bootleader.
!! adb, fast boot do not work!! "power key + down button" dont do anything!
i'm getting a bit resteless because no matter were i search on the web i can not find anything suitable on how to resolve this issue...
there are some youtube videos that show a solution for tf100. it seems the same problem.
there is also a tool for tf100, called easy flasher. to bad it does not work for tf300t
i think it is a question of seaching and waiting. someone will help and get or point to a solution. because if we google for tf300t bricked that are lots and lots of links stating this issue...
what a mess...