Power tubes can last for years, if not decades in a properly-biased class AB amp. What you are more than likely experiencing is AC supply voltage fluctuation, especially if your metro area and surrounding states have experienced a spike in usage during the last few days. Unlike solid-state amps that have regulated power supplies, tube amps are sensitive to AC voltage fluctuations. If you own a multimeter, take wall voltage readings for several days in a row (set the meter to measure AC voltage). I bet that the wall voltage is lower when the amp sounds quieter. Head room is a function of plate voltage, and plate voltage is a function of wall voltage.
By the way, biasing a class AB amp colder increases output power because the amp runs closer to class B, which is a more efficient class of operation. Biasing a class AB amp hotter, pushes it closer to class A operation, which is a very inefficient class of operation. A lot of power is wasted as heat when an amp is biased hot.
