I had a similar problem with an amp that hummed when the line voltage was too high. I called the power company and they came out and retapped the transformer out on the pole to solve the problem. The power company has to adjust your line if it is out of spec so you may wish to pursue that route. Since the weather is now getting cooler, it is only going to get worse with everyone not using their air conditioners.
I would also contact Levinson to see what the high voltage limit is since 126V is not really that high. It is hard to believe that Levinson would build an amp that can't handle AC voltage that is within spec. It seems to me that either the line voltage is out of spec or the 383 is.
You could also try an autotransformer to lower the voltage. They are commonly called Variacs. It is not actually a buck transformer but an adjustable inductive voltage divider. I used one with the amp above before the power company adjusted my voltage and it worked fine. If you live near Nashville I can let you borrow my Variac.
I would also contact Levinson to see what the high voltage limit is since 126V is not really that high. It is hard to believe that Levinson would build an amp that can't handle AC voltage that is within spec. It seems to me that either the line voltage is out of spec or the 383 is.
You could also try an autotransformer to lower the voltage. They are commonly called Variacs. It is not actually a buck transformer but an adjustable inductive voltage divider. I used one with the amp above before the power company adjusted my voltage and it worked fine. If you live near Nashville I can let you borrow my Variac.