Noticed today I get a tea kettle noise when hooked up to my amp. I play a mesa boogie 400+ into a Berg NV610 and NV 215. It happens when I turn the master volume up. I can have the channel volumes at 0 without a bass plugged in and I still get it. It gets worse when I pull the treble knob out. I have my treble usually set at 7. Also, the higher I turn the treble knob, the higher the hiss gets. The higher I turn up the master (even with both channel volumes at zero) worsens the problem as well. I've swapped cabinet cables, and I still have the problem. I'm guessing its a power tube, due to it being aggitated by the master volume knob. (It doesn't just get louder, it gets worse!) Any opinions or suggestions?