Yes Marshall use Svetlanas. My amp is broken (sort of). When playing it sounds fine (cos the noise drowns out the problem), but when there is a guitar plugged in and not playing the noice from the amp sounds like rice crispies on clean, and a thunderstorm on overdrive. The louder you turn up the master volume on the head, the worse the problem is. For gigs, it doesn't really matter butas for recording... Everyone seemed to suggest it was the valves.