Summoming all bass amp knowledge. My buddy's SWR SpellbinderBlue has an inconsistent scratchy tone. I'm trying to figure out what it is. It's not a cap because it won't do it after LONG heavy sustain. It's not a tear in the speaker...moves freely. No obvious issues inside or out. I'm not a tube guy....could a tube cause an intermittent issue like this? Could it be as simple as the Gain pot i was turning in the vid?