Test the amp by touching the tip of the cord you unplug from the guitar and preferably try it with a second cable as well. If at high gain you don't get a loud hum, best to take unit for service. If it hums, then check battery in your guitar. The battery in the guitar is enabled when a cable is plugged in and MANY forget to unplug the cable from the guitar and have a dead battery within a day and blame the amp. This applies to most guitars that have preamps in them... there are some that are "passive" that do not have a battery.
I had a very similar problem with my Spider III 30. Everything powered up but only a slight buzzing noise could be heard at full volume. Try pulling the guitar cable plug out slightly after it seats. You may have to work it a little to find the contact point. It worked like a charm for me. Eventually, I'll open it up and adjust the contacts.
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