Hi I bought an Ampeg SVT-VR amplifier from thomann.de earlier this fall (September), Serial number: efxuu70055. Initially I'm impressed by the sound and overall quality. However during long rehearsals I've begun to experience what I'd call "erratic" behaviour from the amp. First major rehearsal: After approx. 2 hours of playing it was time for a break so I switched the amp to standby. As I switched it back on the output of channel 1 was almost etirely gone. channel 2 worked as normal. I powered down the amp and let i rest for 10 minutes. When I powered it up again the output of channel 1 was back to normal. I experienced the same problem the next day. Third major rehearsal The amp worked perfectly for one and a half day, approx. 12hours of playing at high volume. As I set it to standby the second day the amp lost output in both channels. When going back from standby there was a cascading "fsshh" sound. Powering the down the amp and on again did not correct the problem. As I got back from rehearsals i turned the amp back on, it worked like a charm. This time I only tried it at lover volumes (student housing). I've been careful running the head from grounded outlets only. I have correct impedance on cab (ashdown 414t deep) running proper cables and been careful in moving, warming up and overall usage of the amp. The balance light and bias lights show no problems. Theres been no warning light going from standby into "on". Any immediate diagnosis? Bad tubes? Any tips appreciated!