Hello, Purchased a used USA made SVT classic locally a while back. Great amp as we all know. The amp sounded as it should and both green LED lights lit up on the back of it when I first got it. Now one of the green LEDs does not light up. I allowed it to warm up for 30 min of playing time as noted in the manual, but I am unable to get that second green light to light up no matter how I adjust either bias knob. Does that mean I need a new matched triad of power tubes on that side or both sides? As far as tone goes the amp might be a little less "tight" in the low end than it previous was, but it does not sound bad. My other tube amps, when it was time to replace the power tubes, you could hear it. I'm not necessarily experiencing that here. Should I re-tube the power section, or am I not missing something on the biasing. For other SVT owners, have you found them easy to bias? Has the bias needed to be changed as your tubes got older?? Did you experience an extremely noticeable change in the tone of the amp when they needed biasing or replacing? Looking for advice and experience from my fellow SVT owners. Thanks guys !!!!