Old tubes will just fade away many times until they flare a little on turn on and ___. The amp will sound weak/thin/lame before this happens usually.
When tubes "blow", many times it may be the amp's fault. If I blow a power tube that I know tested good, I would usually want a tech to check it out before I waste a new set of tubes. You don't want to replace just one.
However, tubes are like light bulbs and just blow sometimes. I've seen them not do any harm and I've seen them damage and amp in the process also. I've never experienced a tube hurting any other gear than the amp it's in.
When tubes "blow", many times it may be the amp's fault. If I blow a power tube that I know tested good, I would usually want a tech to check it out before I waste a new set of tubes. You don't want to replace just one.
However, tubes are like light bulbs and just blow sometimes. I've seen them not do any harm and I've seen them damage and amp in the process also. I've never experienced a tube hurting any other gear than the amp it's in.