So.... I'm about to buy a Ampeg SVT-4 Pro head but I have some questions. Lets say I play on it maybe twice a week. How often would i have to replace the tubes? A guy on musicians friend said not to put this head in a rack (even though it is rack ready) he said it didn't get enough airflow... but I really want to get a rack and this be a part of it... what do you think about that?