I bought a brand new DSL 100 in Feb 2019. Before I had to go back to work I bought 2 preamp tubes and tried them with no positive results. When I strum the strings on my guitar it just makes some scratchy noise. I just measured the plate voltage on all 4 power tubes, 437 volts, grid voltage is -35 volts and filament voltage is good. Could it be one or more power tubes is bad?