The best thing to do is not to wonder why both amplifiers failed at the same time. The fact is that both amplifiers need repair, and the best approach to that is to fix each amplifier one at a time. In the process of doing so, it may become obvious why they both failed at the same time, or you may never figure that out. It makes no difference either way.
A transformer coupled tube amplifier operates on certain basic rules. The stress on the output tubes is determined by the plate voltage, the bias voltage, and the current drawn from plate to cathode. The easiest thing that any good technician can do is to determine those parameters for your amplifiers. If any of the 3 parameters is way out of specification, then you have a reason why the tube may have blown, and it's easy to fix. For example, a coupling capacitor between the driver stage and the output tubes may have failed such that there is no bias voltage getting to the grid. This causes the tube to draw maximum current and burn out rapidly. Did you notice the tube glowing red instead of typical orange? I admit it is unlikely to lose a coupling capacitor simultaneously in two different amplifiers, but don't assume that both amplifiers failed for the same reason. For another example, a plate resistor may have failed. This would also cause excessive current draw, a brief red glow, and tube death. I don't see why a competent tech won't look at the circuit, even without a schematic, just to test for these easily diagnosed issues and others like it. If one guy won't look at it, try another. Also, check on-line for a schematic. Also, post this on a tube amplifier forum, e.g., TubeDIY Asylum, etc.