If the hum level changes from time to time, it may be from DC offset in your power line caused by other things in your home contaminating your power. You can try powering the amp from a different circuit using extension cords, or turning off each circuit until the hum stops. Depending on when the offending equipment is in use, the hum level will change which may be an indication that the cause is in your power delivery to the amp. Some transforms are more susceptible to this problem than others, which may account for why this is only happening to this amp. Toroidal (round shaped) power transformers are more apt to hum from less than ideal power feed. The problem with DC offset is that one cannot easily measure it to get a definitive diagnosis and most power conditioners don't deal with it.
Another possibility is that the power transformer is overheating because of a problem in your amp or because the manufacturer put in one that is not up to the task. Is the transformer hot to the touch or merely slightly warm? If it is fairly hot, you should talk with the manufacturer even if you had decided you could live with the slight hum because there could be a bigger problem.