What everyone else said. Also, if you bought the amps used with used tubes installed, it may not be unusual at all to have lost two tubes over a period of 1.5 years. The fact that you do not use the amps so much is neither here nor there. Tubes are most stressed during turn on and warm up.. Just like an electric lightbulb, they are most apt to blow at start up. Especially if you have never checked and adjusted bias during your period of ownership. As tubes age, the bias must be periodically adjusted. However, the tendency during the aging process is to lose transconductance, which means that as the bias stays the same the current through the tube will go down. That per se would not stress the tube; it would cause the sound to grow dull and lifeless.
