Wow, 11 tube amp! 
If you still keep testing it turm voltage selector to highest available. Perhaps it is 250 or 260V? This drop secondary voltages and it should calm down this erratic operation. It is not unusual that there comes hum from speaker when amp is turned on. There flow higher current and hum can transfer to OT elecgromagnetically.
When amp begin to hiss I think is a sign that tubes have heated hot enough and voltages have stabilized and this hissing symptom can begin. It did go quiet soon? Did you turn amp Off or did it by itself? If it did turn quiet by itself does it function? If it does function normally it can just be one of the amp warming up features and possibly you have to accept it? Possibly just one tube make it?
If you still keep testing it turm voltage selector to highest available. Perhaps it is 250 or 260V? This drop secondary voltages and it should calm down this erratic operation. It is not unusual that there comes hum from speaker when amp is turned on. There flow higher current and hum can transfer to OT elecgromagnetically.
When amp begin to hiss I think is a sign that tubes have heated hot enough and voltages have stabilized and this hissing symptom can begin. It did go quiet soon? Did you turn amp Off or did it by itself? If it did turn quiet by itself does it function? If it does function normally it can just be one of the amp warming up features and possibly you have to accept it? Possibly just one tube make it?