If the brightness is harsh, sounds like crap, your tubs are at the onset of HF oscillation. The hum is the noise you get when the tubes begin to red plate (fully saturated by uncontrolled oscillation which tends to operate the plate current above maximum ratings, typical sign on failing tubes). In 90W mode, V10 and V8 are operating in parallel (same applies for V9 and V11). I had this issue with my amp within 2 months of ownership. If you power it up again, seems fine right? Tubes are on the way out. If you play through the amp again, and get that same noise, look at the tubes first before dropping the stanby switch. Your reaction is the best choice since failing tubes can also cause other issues depending on how they go on you. As for the poping noise, it will probably be very loud and you may even get bright flash when you kill the signal path of all the tubes. That is due to sudden change in current flow throught the transformers (primarily the OT).
