Hey, I recently bought this amp (5 months ago) and it was working very well until a couple of days ago. Yesterday I turned on the amp and it's volume was very low on both channels. I had to put it all on maximum in the classic gain channel (Volume and Gain) to get a decent volume for practicing on my bedroom. I reseated all valves, but the problem continues. I also tested the valves hitting them with a pen, but there was no sound that could indicate a bad valve. What should I do?