I have the ottos box xp stone and got my toon to Lv 8 only to find I cant use it now that I'm level 8. Ahh! When they first showed up you could use them at lv 8, but now (my fault for not reading the fine rune print) "You must be below the stones level" to use it. Now I have to run the full level to 20 to do my Next TR..
So just a FYI, make sure you use the xp Rune at lv 7 or suffer the grind.