So I flip on the power on my DB 359, flip the standby swith start playing and then...sound goes dead, power is off and tubes start "tinkling." I look at the back of the amp and over the vent where one tube is, it's really warm. After a minute of disbelief, I open up the case and remove all 4 tubes. The tube that was in the area of the warmth has some blackish discoloration on it. It's not too bad, but none of the others have anything like that. I can't see anything busted in there, but I'm pretty much convinced that I blew this tube. Can anyone confirm and if so, point me in the right direction to replace it/them for as cheaply as possible? Thanks.