"You have severely stressed the output tubes by turning the amplifier on without a load."
This sounds like a straight-up guess. In any case, it's completely wrong.
A tube amp's *output transformers* can be damaged (overheated) if the amp is *played* with no load. If there is no SIGNAL (as the OP states), then there is NO problem. In either case, there is no such thing as "stress" to the tubes.
Think about it: When the speaker wires are connected, there is nothing happening if there is no input signal to the amp - if there WAS something happening - some voltage present - you'd hear it out the speakers. So how could merely disconnecting those leads make any difference to the amp?
This sounds like a straight-up guess. In any case, it's completely wrong.
A tube amp's *output transformers* can be damaged (overheated) if the amp is *played* with no load. If there is no SIGNAL (as the OP states), then there is NO problem. In either case, there is no such thing as "stress" to the tubes.
Think about it: When the speaker wires are connected, there is nothing happening if there is no input signal to the amp - if there WAS something happening - some voltage present - you'd hear it out the speakers. So how could merely disconnecting those leads make any difference to the amp?