Is it a problem or do all DSL 100's when turned all the way down still play quietly on the lead channels? If so I guess i'll just call it sweet Marshall bonus sound.
Yup, it's a "feature" of most high gain amps. Even with the dial at 0 the signal wont be at absolute ground potential and a small signal voltage will be present. Amps with high enough gain levels will pcik this up and amplify it.