Anyone have any idea how long power tubes should last in a class-A, EL-84 bassed amp design, running about 2 to 3 hours nightly? I'm getting about 3 months out of my tubes... although I'm a little concerned that it could be something else. The amp is sounding dull, and the volume drops significantly with some crackling like noise. Athough it will work fine for about 15 minutes when I first turn her on. Does this sound like the tubes need to be swapped or something more serious.. given that I swapped the power tubes about 2.5 months ago and preamp/inverter about 6 months ago.