I have a DSL40C i purchased in September. After a few weeks it started to emmit an arcing sound when in standby. I returned it to the supplier who found nothing wrong. It has been ok since except for a  wooshing sound when it is first turned on that stays at the same volume even if I turn the volume up but slowly dissapears over time (I assumed this was just the valves heating up and settling and happened in both normal and low power modes) Today it started the arcing again and it would not turn the volume off in standby mode, then standby mode started working correctly and now if i put the amp in low power mode the volume starts decreasing and eventually disappears after about 10 seconds. Normal power mode seems to be OK though. Does this point to defective tubes already?