The description for a Staff of Power specifically states that, "After all charges are used up from the staff, it remains a +2 quarterstaff." It doesn't say what happens to the +2 Luck bonus to AC and saves. On one hand, the description doesn't explicitly state that they continue to exist after all the charges are gone, but on the other hand, there's no reason that using charges should diminish that aspect of the staff's power (the same way it doesn't diminish the enhancement bonus to attack and damage). What do you think? Should a "used up" Staff of Power keep the AC and save bonus or not?