If this is the first and only set of tubes you had, my first instinct would be a faulty production batch from the factory. I wouldn't suspect the amplifiers unless the problem happened with one amp. Moreover, a bias current of 55 to 60mA means the amp is running in Class AB with a B+ voltage around 550V for a 100 watt rated output. At this condition the KT150's are dissipating only 50% of their rated power. This should make the tube last way longer than the 1,000 hour life quoted by Tung Sol.
That said, there are a couple of things a competent tech can do without opening the hood. Power up the amps without the tubes and measure the voltage on pin 5 on each KT150 socket to check the grid voltage. All four grids should have the same voltage within a few hundred millivolts. The pcb pictures of this amp show a time delay to turn on the B+, it is critical -- critical -- that the bias voltage appears on the grid before the B+ turns on and the B+ voltage drops faster than the bias voltage when powered off. Having a substantial voltage on the plate without a bias voltage drives the tube into high current and shortens the tube life considerably.
My suggestion would be to buy a set of tubes from Upscale Audio where they claim to weed out weak factory batches and run the tubes hard prior to sale.