HenDog -- Twelve and a half hours and 173 is just about right. Your butt was just coming out of the plateau. I think if you would have left it at 225 more of the fat would have rendered out. Once out of the plateau she climbs fairly quick. Patience is a key!
For what it's worth, I have got into the habit of starting my butts the night before, setting the temp at 190-200. Get up in the morning, check the internal temp and crank the smoker up to 225. Can almost always guaranteee lunch by noon.
Let us know how youur smoke turned out...