The Mesa techs near you are trying to take you for a ride. It's not very professional of them to recommend you change the PT without first checking it (and though I'm not too familiar with the AUS exchange rate I bet their part quote is more than 300% markup)
It's like waking up in the morning and finding that your car won't start, describing it to a mechanic over the phone, then having him tell you that it'll need a new engine to get it running... all before, say, checking the battery.
It's almost certainly a faulty power tube. Check that there is a negative bias voltage at the control grids just to be sure you don't pop the replacements, then sub the new ones in and make sure they're biased okay.