I would imagine it is coming from the speakers, but a way to check would be to attach lamp cord to the speaker output on the amp (disconnect the original speaker), and use another 8ohm speaker quite a distance away. Then the speaker and the amp will be somewhat isolated. Sometimes if there is a power transformer problem, the transformers will hum if they are under too much load, but I'd imagine your hum is coming from the speaker. Different tubes hum differently, even within a different type. If your input tube is a 12ax7, you could try a 12ay7 to reduce gain at the input and input tube hum (12ay7 is a low-noise low-hum type). If it's a pentode like ef86, try using different ef86 tubes, some are better than others. Long guitar cables hum way more than short ones, and long extension cords can introduce hum into the power supply. Try turning the volume off on the instrument, if hum persists, it's probably the cable or the amp--guitar strings are essentially antennae.