"Radio Shack - You've got questions, we've got answers". Wrong answers but answers none the less. I'd call VTL for answers if I were you.
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The amp rating of the line that goes to any outlet is a rating of how much current that line can carry before its protection - thru a breaker typically - will trip and shut down the circuit to protect from damage (usually meaning it can start a fire).
This is completely independent of the amp rating of what is upstream of the breaker - lets say the 200amp breaker that protects your entire breaker panel and service into your home.
In the same way, the fuse on your amp protects it from exceeding the maximum current it can withstand without causing internal damage to the amp, and like the breaker example, is completely independent of what is upstream. After all, it was not a 15amp fuse when you had a 15amp circuit.
When tubes age, they tend to draw higher current peaks. When you changed to the independent circuit to the amps, you gave the amps more current availability as nothing else was consuming current at the same time on that same circuit, which pushed those voltage peaks up slightly (the increase in dynamics you likely heard) and started popping fuses.
With your 15amp curcuit the problem was likely coming soon, with just a little more degradation of the tube.